Exhibit 4(a)(6) Supplemental Indenture, dated as of September 1, 1995 (the "Supplemental Indenture"), made by and between Northwestern Public Service Company, a corporation organized and existing under the laws of the State of Delaware (the "Company"), the post office address of which is 33 Third Street, S.E., Huron, South Dakota 57350, and The Chase Manhattan Bank (National Association), a national banking association organized and existing under the laws of the United States of America (the "Trustee"), as Trustee under the General Mortgage Indenture and Deed of Trust dated as of August 1, 1993, hereinafter mentioned, the post office address of which is 4 Chase MetroTech Center, 3rd Floor, Brooklyn, New York 11245; Whereas, the Company has heretofore executed and delivered its General Mortgage Indenture and Deed of Trust dated as of August 1, 1993 (the "Original Indenture"), to the Trustee, for the security of the Bonds of the Company issued and to be issued thereunder (the "Bonds"); and Whereas, the Company has heretofore executed and delivered to the Trustee certain indentures supplemental to the Original Indenture dated as of August 15, 1993 and as of August 1, 1995 and, contemporaneously herewith, the Company has executed and delivered to the Trustee two certain indentures dated as of September 1, 1995 relating to New Mortgage Bonds of the 8.90% Series and New Mortgage Bonds of the 6.99% Series, respectively (the Original Indenture, as supplemented and amended by the aforementioned supplemental indentures and by this Supplemental Indenture being hereinafter referred to as the "Indenture"); and Whereas, the Company desires to create a new series of Bonds to be issued under the Indenture, to be known as New Mortgage Bonds, 8.824% Series due 1998 (the "New Mortgage Bonds of the 8.824% Series"), which New Mortgage Bonds of the 8.824% Series are to be issued in exchange for certain other bonds of the Company of like tenor and amount that were issued pursuant to a supplemental indenture to the Company's Indenture dated August 1, 1940; and Whereas, the Company, in the exercise of the powers and authority conferred upon and reserved to it under the provisions of the Indenture, and pursuant to appropriate resolutions of the Board of Directors, has duly resolved and determined to make, execute and deliver to the Trustee a Supplemental Indenture in the form hereof for the purposes herein provided; and Whereas, all conditions and requirements necessary to make this Supplemental Indenture a valid, binding and legal instrument have been done, performed and fulfilled and the execution and delivery hereof have been in all respects duly authorized; Now, Therefore, This Indenture Witnesseth: That Northwestern Public Service Company, in consideration of the exchange referred to above and ownership from time to time of the Bonds and the service by the Trustee, and its successors, under the Indenture and of One Dollar to it duly paid by the Trustee at or before the ensealing and delivery of these presents, the receipt whereof is hereby acknowledged, hereby covenants and agrees to and with the Trustee and its successors in the trust under the Indenture, for the benefit of those who shall hold the Bonds as follows: Article I Description Of Bonds Of The 8.824% Series Due 1998 Section 1. The Company hereby creates a new series of Bonds to be known as "New Mortgage Bonds, 8.824% Series due 1998." The New Mortgage Bonds of the 8.824% Series shall be executed, authenticated and delivered in accordance with the provisions of, and shall in all respects be subject to, all of the terms, conditions and covenants of the Indenture, as supplemented and modified. The commencement of the first interest period for the New Mortgage Bonds of the 8.824% Series shall be July 15, 1995. The New Mortgage Bonds of the 8.824% Series shall mature July 15, 1998, and shall bear interest at the rate of 8.824% per annum, payable semi-annually on the fifteenth day of January and the fifteenth day of July in each year. The person in whose name any of the New Mortgage Bonds of the 8.824% Series are registered at the close of business on any record date (as hereinafter defined) with respect to any interest payment date shall be entitled to receive the interest payable on such interest payment date notwithstanding the cancellation of such New Mortgage Bonds of the 8.824% Series upon any transfer or exchange subsequent to the record date and prior to such interest payment date; provided, however, that if and to the extent the Company shall default in the payment of the interest due on such interest payment date, such defaulted interest shall be paid as provided in Section 3.07(b) of the Indenture. The term "record date" as used in this Section with respect to any interest payment date shall mean January 1 or July 1, as the case may be, next preceding the semi-annual interest payment date, or, if such January 1 or July 1 shall be a legal holiday or a day on which banking institutions in the Borough of Manhattan, The City of New York, State of New York, are authorized by law to close, then the next preceding day which shall not be a legal holiday or a day on which such institutions are so authorized to close. Section 2. The New Mortgage Bonds of the 8.824% Series shall be issued only as registered Bonds without coupons of the denomination of $1,000, or any integral multiple of $1,000, appropriately numbered. The New Mortgage Bonds of the 8.824% Series may be exchanged, upon surrender thereof, at the agency of the Company in the Borough of Manhattan, The City of New York, State of New York, for one or more new New Mortgage Bonds of the 8.824% Series of other authorized denominations, for the same aggregate principal amount, subject to the terms and conditions set forth in the Indenture. New Mortgage Bonds of the 8.824% Series may be exchanged or transferred without expense to the registered owner thereof except that any taxes or other governmental charges required to be paid with respect to such transfer or exchange shall be paid by the registered owner requesting such transfer or exchange as a condition precedent to the exercise of such privilege. Section 3. The New Mortgage Bonds of the 8.824% Series and the Trustee's Certificate of Authentication shall be substantially in the following forms respectively: [Form Of Bond Of The 8.824% Series Due 1998] Northwestern Public Service Company (Incorporated under the laws of the State of South Dakota) New Mortgage Bond, 8.824% Series Due 1998 No. R- $______________ Northwestern Public Service Company, a corporation organized and existing under the laws of the State of Delaware (the "Company", which term shall include any successor corporation as defined in the Indenture hereinafter referred to), for value received, hereby promises to pay to __________ or registered assigns, the sum of __________ dollars on the fifteenth day of July, 1998, in any coin or currency of the United States of America which at the time of payment is legal tender for public and private debts, and to pay interest thereon in like coin or currency from July 15, 1995, payable semi-annually, on the fifteenth days of January and July in each year, at the rate of 8.824% per annum, until the Company's obligation with respect to the payment of such principal shall be discharged as provided in the Indenture hereinafter mentioned. The interest so payable on any January 15 or July 15 will, subject to certain exceptions provided in the Supplemental Indenture dated as of September 1, 1995, be paid to the person in whose name this Bond is registered at the close of business on the immediately preceding January 1 or July 1, as the case may be. Both principal of, and interest on, this Bond are payable at the agency of the Company in the Borough of Manhattan, The City of New York, State of New York. This Bond shall not be entitled to any benefit under the Indenture or any indenture supplemental thereto, or become valid or obligatory for any purpose, until the form of certificate endorsed hereon shall have been signed by or on behalf of The Chase Manhattan Bank (National Association), the Trustee under the Indenture, or a successor trustee thereto under the Indenture, or by an authenticating agent duly appointed by the Trustee in accordance with the terms of the Indenture. The provisions of this New Mortgage Bond are continued on the reverse hereof and such continued provisions shall for all purposes have the same effect as though fully set forth at this place. In Witness Whereof, Northwestern Public Service Company has caused this New Mortgage Bond to be signed (manually or by facsimile signature) in its name by an Authorized Executive Officer, as defined in the Indenture, and its corporate seal (or a facsimile thereof) to be hereto affixed and attested (manually or by facsimile signature) by an Authorized Executive Officer, as defined in the Indenture. Dated: Northwestern Public Service Company By Authorized Executive Officer ATTEST: Authorized Executive Officer [Form of Trustee's Certificate] This is one of the Bonds of the series designated therein referred to in the within-mentioned Indenture and Supplemental Indenture dated as of September 1, 1995. The Chase Manhattan Bank (National Association), as Trustee By Authorized Officer [FORM OF REVERSE OF BOND] This New Mortgage Bond of the 8.824% Series is one of a duly authorized issue of Bonds of the Company (the "Bonds"), of the series hereinafter specified, all issued and to be issued under and equally secured by a General Mortgage Indenture and Deed of Trust (the "Indenture"), dated as of August 1, 1993, executed by the Company to The Chase Manhattan Bank (National Association) (the "Trustee"), as Trustee, to which Indenture and all indentures supplemental thereto reference is hereby made for a description to the properties mortgaged and pledged, the nature and extent of the security, the rights of registered owners of the Bonds and of the Trustee in respect thereof, and the terms and conditions upon which the Bonds are, and are to be, secured. The Bonds may be issued in series, for various principal sums, may mature at different times, may bear interest at different rates and may otherwise vary as provided in the Indenture. This New Mortgage Bond of the 8.824% Series is one of a series designated as the "New Mortgage Bonds, 8.824% Series Due 1998" (the "New Mortgage Bonds of the 8.824% Series") of the Company issued under and secured by the Indenture and described in the supplemental indenture dated as of September 1, 1995 (the "Supplemental Indenture dated as of September 1, 1995"), between the Company and the Trustee, supplemental to the Indenture. New Mortgage Bonds of the 8.824% Series, of which this is one, are subject to redemption at the option of the Company and upon the notice and in the manner and with the effect provided in the Indenture, all, but not less than all, of the New Mortgage Bonds of the 8.824% Series may be redeemed by the Company at any time, on or after July 15, 1996 and prior to maturity, upon payment of the following percentages of the principal amounts thereof: If redeemed during the twelve month period beginning the fifteenth day of July of the year: 1996 ---------- 101.471% 1997 ---------- 100.000% together with accrued interest to the redemption date. To the extent permitted by, and as provided in, the Indenture, modifications or alterations of the Indenture, or of any indenture supplemental thereof, and of the rights and obligations of the Company and of the holders of the Bonds may be made with the consent of the Company by an affirmative vote of the holders of a majority in aggregate principal amount of the Bonds entitled to vote then outstanding, at a meeting of the holders of the Bonds called and held as provided in the Indenture, and by an affirmative vote of the holders of a majority in aggregate principal amount of the Bonds of any series or any tranche or tranches of any series entitled to vote then outstanding and affected by such modification or alteration, in case one or more but less than all of the series of Bonds or of any tranche or tranches of any series of Bonds then outstanding under the Indenture are so affected; provided, however, that no such modification or alteration shall be made which will affect the terms of payment of the principal of, or interest or premium, if any, on this Bond. In case an Event of Default, as defined in the Indenture, shall occur, the principal of all the New Mortgage Bonds of the 8.824% Series at any such time outstanding under the Indenture may be declared or may become due and payable, upon the conditions and in the manner and with the effect provided in the Indenture. The Indenture provides that such declaration may be rescinded under certain circumstances. Article II Issue Of New Mortgage Bonds Of The 8.824% Series Section 1. The Company hereby exercises the right to obtain the authentication of $15,000,000 principal amount of Bonds pursuant to the terms of Section 4.03 of the Indenture. All such Bonds shall be New Mortgage Bonds of the 8.824% Series. Section 2. Such New Mortgage Bonds of the 8.824% Series may be authenticated and delivered prior to the filing for recordation of this Supplemental Indenture. Article III Redemption Section 1. The New Mortgage Bonds of the 8.824% Series shall, upon the notice and in the manner and with the effect provided in Article Five of the Original Indenture, as amended by Section 2 of this Article III, be redeemable at any time, on or after July 15, 1996 and prior to maturity, at the option of the Company, as a whole, upon payment of the following percentages of the principal amounts thereof: If redeemed during the twelve month period beginning the fifteenth day of July of the year: 1996 ---------- 101.471% 1997 ---------- 100.000% together with accrued interest to the redemption date. Section 2. Notice of redemption of any New Mortgage Bonds of the 8.824% Series shall be given as provided in Section 5.04 of the Original Indenture. If given by mail, the mailing of such notice shall be a condition precedent to redemption, provided that any notice which is mailed in the manner provided in Section 5.04 of the Original Indenture shall be conclusively presumed to have been duly given whether or not the holders receive such notice, and failure to give such notice by mall, or any defect in such notice, to the holder of any such bond designated for redemption shall not affect the validity of the redemption of any other such bond. Except for the changes in the giving of notice of redemption as provided in this Section, the procedures for redemption of the New Mortgage Bonds of the 8.824% Series shall be as provided in Article Five of the Original Indenture. Section 3. The Company, with the approval of the Trustee, may enter into a written agreement with the holder of any New Mortgage Bonds of the 8.824% Series providing that payment of such bonds called for redemption in part only be made directly by mail, wire transfer or in any other manner to the holder thereof without presentation or surrender thereof if there shall be delivered to the Trustee an agreement (which may be a composite with other such agreements) between the Company and such holder (or other person acting as agent for such holder or for whom such holder is a nominee) that payment shall be so made, and that in the event the holder thereof shall sell or transfer any such bonds (a) it will, prior to the delivery of such bonds, either (i) surrender such bonds to the Trustee to make a proper notation of the amount of principal paid thereon or (ii) surrender such bonds to the Trustee against receipt of one or more New Mortgage Bonds of the 8.824% Series in an aggregate principal amount equal to the unpaid principal portion of the bonds so surrendered, and (b) it will promptly notify the Company of the name and address of the transferee of any New Mortgage Bonds of the 8.824% Series so transferred. The Trustee shall not be liable or responsible to any such holder or transferee or to the Company or to any other person for any act or omission to act on the part of the Company or any such holder in connection with any such agreement. The Company will indemnify and save the Trustee harmless against any liability resulting from any such act or omission and against any liability resulting from any action taken by the Trustee in accordance with the provisions of any such agreement. Article IV The Trustee The Trustee hereby accepts the trusts hereby declared provided, and agrees to perform the same upon the terms and conditions in the Indenture set forth and upon the following terms and conditions: The Trustee shall not be responsible in any manner whatsoever for or in respect of the validity or sufficiency of this Supplemental Indenture or the due execution hereof by the Company or for or in respect of the recitals contained herein, all of which recitals are made by the Company solely. In general, each and every term and condition contained in Article Eleven of the Indenture shall apply to this Supplemental Indenture with the same force and effect as if the same were herein set forth in full, with such omissions, variations and modifications thereof as may be appropriate to make the same conform to this Supplemental Indenture. Article V Miscellaneous Provisions This Supplemental Indenture may be simultaneously executed in any number of counterparts, each of which when so executed shall be deemed to be an original; but such counterparts shall together constitute but one and the same instrument. In Witness Whereof, said Northwestern Public Service Company has caused this Indenture to be executed on its behalf by an Authorized Executive Officer as defined in the Indenture, and its corporate seal to be hereto affixed and said seal and this Indenture to be attested by an Authorized Executive Officer as defined in the Indenture; and The Chase Manhattan Bank (National Association), in evidence of its acceptance of the trust hereby created, has caused this Indenture to be executed on its behalf by its President or one of its Vice Presidents and its corporate seal to be hereto affixed and said seal and this Indenture to be attested by its Secretary or one of its Assistant Secretaries; all as of the 1st day of September, 1995. Northwestern Public Service Company By Vice President (CORPORATE SEAL) ATTEST: Corporate Secretary The Chase Manhattan Bank (National Association) By Vice President (CORPORATE SEAL) ATTEST: Assistant Secretary State Of South Dakota ) ) SS County Of Beadle ) Be it Remembered, that on this 1st day of September, 1995, before me, Susan M. Anderson, a Notary Public within and for the County and State aforesaid, personally came D. K. Newell, Vice President - Finance, and Alan D. Dietrich, Corporate Secretary of Northwestern Public Service Company, a Delaware corporation, who are personally known to me to be such officers, and who are personally known to me to be the same persons who executed as such officers the within instrument of writing, and such persons duly acknowledged that they signed, sealed and delivered the said instrument as their free and voluntary act as such Vice President - Finance and Corporate Secretary, respectively, and as the free and voluntary act of Northwestern Public Service Company for the uses and purposes therein set forth. In Witness Whereof, I have hereunto subscribed my name and affixed my official seal on the day and year last above written. (Notarial Seal) Susan M. Anderson Notary Public, Beadle County, S.D. My Commission expires June 8, 2000 State Of New York ) ) SS County Of Kings ) Be it Remembered, that on this _____ day of August, 1995, before me, _______________, a Notary Public within and for the County and State aforesaid, personally came _________________, Vice President, and _________________, Assistant Secretary of The Chase Manhattan Bank (National Association), a national banking association, who are personally known to me to be such officers, and who are personally known to me to be the same persons who executed as such officers the within instrument of writing, and such persons duly acknowledged that they signed, sealed and delivered the said instrument as their free and voluntary act as such Vice President and Assistant Secretary, respectively, and as the free and voluntary act of The Chase Manhattan Bank (National Association) for the uses and purposes therein set forth. In Witness Whereof, I have hereunto subscribed my name and affixed my official seal on the day and year last above written. (Notarial Seal) Notary Public, State of New York No. __________ Qualified in Kings County Commission expires ________, ____