<Page>
Exhibit 99.5
MONTANA POWER LLC
UNAUDITED PRO FORMA COMBINED CONDENSED FINANCIAL DATA
The following presents unaudited pro forma combined financial data for
the Montana Power, L.L.C. ("Montana Power LLC") as of and for the year ended
December 31, 2001. The pro forma combined condensed statement of income gives
effect to the following as if each transaction had occurred as of January 1,
2001 and the pro forma combined condensed balance sheet gives effect to the
following as if each transaction had occurred on December 31, 2001:
- the exclusion of revenues and costs under an industrial power supply
contract, which was not assumed as part of the acquisition of Montana
Power LLC;
- the elimination of certain regulatory liabilities relating to the sale by
the former owner of Montana Power LLC of its oil and gas businesses, which
liabilities were retained by the former owner of Montana Power LLC and for
which the former owner of Montana Power LLC is fully indemnifying
NorthWestern Corporation ("NorthWestern") as part of the acquisition of
Montana Power LLC;
- the elimination of interest expense on related party and other debt and
interest income on related party notes receivable, which were not assumed
as part of the acquisition of Montana Power LLC; and
- the elimination of dividends paid on The Montana Power Company's preferred
stock, which is being retained by the former owner of Montana Power LLC as
part of the acquisition of Montana Power LLC.
The pro forma adjustments are based upon currently available information and
certain assumptions that our management believes are reasonable. The unaudited
pro forma financial information is prepared for illustrative purposes only and
is not necessarily indicative of the operating results or financial condition of
Montana Power LLC that would have occurred had the transactions occurred at the
periods presented, nor is the unaudited pro forma financial information
necessarily indicative of future operating results or the future financial
position of Montana Power LLC.
Pro forma adjustments include certain cash accounts representing excess
proceeds from the sale by the former owner of Montana Power LLC of its
generation assets, which will be applied to reduce transition and stranded
costs under the jurisdiction of the Montana Public Service Commission
("MPSC") and the Federal Energy Regulatory Commission.
In addition, as a result of legislation enacted in Montana in 2001, recovery
of that portion of power supply costs related to certain qualified facilities
which are currently not recovered in rates, as reflected in the Unaudited Pro
Forma Consolidated Statement of Income, are anticipated to be recovered in rates
in future periods. The Unaudited Pro Forma Consolidated Statement of Income for
the year ended December 31, 2001 reflects an offsetting one-time adjustment
related to the capitalization of a regulatory asset which resulted from the
final settlement of a general rate increase approved by the MPSC on May 8, 2001.
The stipulation agreement approved by the MPSC on January 31, 2002 provided
for a one time credit to customers totaling $30 million which was funded at
closing by the former owner of Montana Power LLC and NorthWestern. Also, under
the Unit Purchase Agreement, the former owner of Montana Power LLC was
responsible for refunds not yet returned to ratepayers related to the former
owner's prior sale of oil and gas properties. The former owner of Montana Power
LLC provided cash at closing in an amount equal to the refunds not yet returned
to ratepayers. Additionally, at closing, the former owner of Montana Power LLC
was required to provide a minimum cash level resulting from the net proceeds of
the generation assets sale completed by it. These proceeds represent the final
gain to be refunded to ratepayers plus funding for certain environmental
liabilities. Montana Power LLC's noncurrent regulatory liability was adjusted to
reflect the net present value of the total refund to ratepayers consisting of
the $60 million agreed to in the stipulation plus the gain from the generation
asset sale completed by the seller.
You should read the information set forth below together with the combined
financial statements and notes thereto of The Utility of The Montana Power
Company and the notes thereto included as Exhibit 99.3 to this Current Report
on Form 8-K.
1
<Page>
MONTANA POWER LLC
UNAUDITED PRO FORMA COMBINED CONDENSED STATEMENT OF INCOME
(IN THOUSANDS)
<Table>
<Caption>
FOR THE YEAR ENDED
DECEMBER 31, 2001
---------------------------------------------------
THE UTILITY PRO FORMA PRO FORMA
ACTUAL(1) ADJUSTMENTS MONTANA POWER LLC
----------- ----------- -----------------
OPERATING REVENUES................................. $703,785 $ (45,655)(2) $658,130
COST OF SALES...................................... 458,777 (131,744)(2) 327,033
-------- --------- --------
GROSS MARGIN....................................... 245,008 86,089 331,097
-------- --------- --------
OPERATING EXPENSES:
Selling, general and administrative expenses....... 191,540 (23,800)(3) 167,740
Depreciation and amortization...................... 56,725 56,725
-------- --------- --------
248,265 (23,800) 224,465
-------- --------- --------
OPERATING INCOME (LOSS)............................ (3,257) 109,889 106,632
Interest expense................................... (39,678) 2,533 (4) (37,145)
Investment income and other........................ 1,355 (1,576)(4) (221)
-------- --------- --------
INCOME (LOSS) BEFORE INCOME TAXES AND MINORITY (41,580) 110,846 69,266
INTERESTS........................................
Benefit (provision) for income taxes............... 4,074 (29,708) (25,634)
-------- --------- --------
NET INCOME (LOSS).................................. (37,506) 81,138 43,632
Minority interest on preferred securities of (5,492) 1,954 (5) (3,538)
subsidiary trust.................................
Dividends on cumulative preferred stock............ (3,770) 3,770 (6)
-------- --------- --------
EARNINGS (LOSS) ON LLC UNITS....................... $(46,768) $ 86,862 $ 40,094
======== ========= ========
</Table>
The accompanying notes are an integral part of these unaudited pro forma
combined condensed financial statements.
2
<Page>
MONTANA POWER LLC
UNAUDITED PRO FORMA COMBINED CONDENSED BALANCE SHEET
(IN THOUSANDS)
<Table>
<Caption>
AT DECEMBER 31, 2001
---------------------------------------------------------------------------------
REMOVE
PREFERRED STIPULATION
STOCK AND AND RATE
THE UTILITY RELATED PARTY DEBT ORDER PRO FORMA
ACTUAL(1) BALANCES(7) ADJUSTMENT(8) ADJUSTMENT(9) MONTANA POWER LLC
----------- ------------- ------------- ------------- -------------------
ASSETS
CURRENT ASSETS:
Cash and cash equivalents...... $ 7,510 $ (7,510) $(19,799) $ 81,988 $ 62,189
Restricted cash................ 30,000 30,000
Accounts receivable, net....... 120,687 (41,845) 78,842
Inventories.................... 10,760 10,760
Other current assets........... 69,178 (31,139) 38,039
---------- -------- -------- --------- ----------
208,135 (80,494) (19,799) 111,988 219,830
PROPERTY, PLANT AND EQUIPMENT,
NET............................ 1,091,235 19,799 1,111,034
GOODWILL AND OTHER INTANGIBLE
ASSETS, NET.................... 7,418 7,418
OTHER ASSETS:
Investments.................... 25,936 25,936
Other assets................... 230,175 (100,423) 129,752
---------- -------- -------- --------- ----------
$1,562,899 $(80,494) $ -- $ 11,565 $1,493,970
========== ======== ======== ========= ==========
LIABILITIES AND SHAREHOLDERS'
EQUITY
CURRENT LIABILITIES:
Current maturities of long-term
debt......................... $ 16,061 $(12,666) $ 3,395
Short-term debt................ 24,811 $(24,811)
Accounts payable............... 64,806 (39,983) 24,823
Dividends payable.............. 776 (776)
Regulatory liability........... 22,166 $ 35,721 57,887
Other.......................... 82,360 82,360
---------- -------- -------- --------- ----------
210,980 (65,570) (12,666) 35,721 168,465
---------- -------- -------- --------- ----------
LONG-TERM LIABILITIES:
Long-term debt................. 442,680 442,680
Deferred income taxes.......... 49,333 (49,333)
Regulatory liability........... 223,423 (100,423) 123,000
Other noncurrent liabilities... 190,713 (12,718) 177,995
---------- -------- --------- ----------
906,149 (62,051) (100,423) 743,675
---------- -------- --------- ----------
Company obligated mandatorily
redeemable securities of trust
holding solely parent
debentures..................... 65,000 (23,121) 41,879
SHAREHOLDERS' EQUITY:
Preferred stock................ 57,654 (57,654)
Other equity................... 323,116 104,781 35,787 76,267 539,951
---------- -------- -------- --------- ----------
380,770 47,127 35,787 76,267 539,951
---------- -------- -------- --------- ----------
$1,562,899 $(80,494) $ -- $ 11,565 $1,493,970
========== ======== ======== ========= ==========
</Table>
The accompanying notes are an integral part of these unaudited pro forma
combined condensed financial statements.
3
<Page>
MONTANA POWER LLC
NOTES TO UNAUDITED PRO FORMA COMBINED
CONDENSED FINANCIAL DATA
The Unaudited Pro Forma Combined Condensed Financial Data of Montana Power LLC
are based on the following assumptions:
(1) Montana Power LLC reflects the electric and natural gas transmission and
distribution utility operations and certain unregulated, energy-related
businesses, including Colstrip Unit 4, of The Utility of The Montana
Power Company that provide products and services to industrial,
institutional and commercial customers. These operations and businesses
are discussed in the audited financial statements for The Utility of The
Montana Power Company included elsewhere herein. These entities represent
the entities which were part of the sale to NorthWestern and are
collectively referred to as Montana Power LLC.
(2) Reflects the elimination of $45.7 million of revenues and
$131.7 million of associated power supply and other costs for the year
ended December 31, 2001 under an industrial power supply contract.
Pursuant to the Unit Purchase Agreement, these associated revenues and
costs have been excluded. The Unit Purchase Agreement required the power
supply contract and all obligations related thereto to remain with the
former owner of Montana Power LLC and is not part of Montana Power LLC
that was acquired by NorthWestern. Montana Power LLC has terminated two
other supply contracts which had combined revenues of $4.3 million and
associated power supply costs of $8.6 million for the year ended
December 31, 2001. Because these contracts were not specifically required
to be terminated by the Unit Purchase Agreement, the pro forma statement
of income has not been adjusted to exclude the losses on these contracts.
(3) On October 31, 2000, The Utility of The Montana Power Company sold its
oil and natural gas businesses. As a result of the transaction, The
Utility of The Montana Power Company recorded a regulatory liability of
$32.5 million in the fourth quarter of 2000 representing the portion of
the proceeds from the sale of oil and natural gas businesses, which The
Utility of The Montana Power Company believed was attributable to
properties previously included in the natural gas utility rate base. An
additional $23.8 million liability was recorded in the third quarter of
2001. NorthWestern is specifically indemnified, pursuant to the Unit
Purchase Agreement, from any impact relating to the regulatory treatment
of the gain on the sale of the oil and gas businesses. Therefore, the pro
forma adjustment reflects the reversal of the $23.8 million charge in the
year ended December 31, 2001.
(4) Reflects the elimination of $2.1 million of interest expense on related
party debt and $0.4 million of interest expense on other debt and
$1.6 million of interest income on related party notes receivable, which
were not assumed as part of the acquisition of Montana Power LLC.
(5) Reflects the reduction in minority interest on preferred securities of
subsidiary trust of $2.0 million resulting from the planned redemption of
$23.1 million of company obligated mandatorily redeemable securities of
trust holding solely parent debentures from cash received upon the
closing of the Unit Purchase Agreement to reduce debt and preferred stock
outstanding to the $488 million required by the Unit Purchase Agent.
(6) Reflects the elimination of preferred stock which was not assumed by
NorthWestern pursuant to the Unit Purchase Agreement and $3.8 million of
dividends for the year ended December 31, 2001 on such preferred stock.
4
<Page>
MONTANA POWER LLC
NOTES TO UNAUDITED PRO FORMA COMBINED (CONTINUED)
CONDENSED FINANCIAL DATA
(7) Reflects adjustments to remove assets and liabilities related to
preferred stock, related party debt and income tax balances of The
Utility of The Montana Power Company which were not acquired as part of
the acquisition.
(8) Pursuant to the Unit Purchase Agreement with NorthWestern, which
required that NorthWestern assume no more than $488 million in Montana
Power LLC debt and preferred securities, the Unaudited Balance Sheet
adjusts company obligated mandatorily redeemable securities of trust
holding solely parent debentures so that the aggregate amount of Montana
Power LLC debt and preferred securities assumed by NorthWestern upon
closing of the acquisition equaled $488 million. In addition,
$19.8 million of cash was used at closing to buy out a synthetic lease
obligation for automated meter reading equipment.
(9) Reflects the estimated impact to regulatory liabilities related to the
stipulation agreement approved by the MPSC, which provided for a one time
credit to customers totaling $30.0 million to be funded by the former
owner of Montana Power LLC and NorthWestern. Also, under the Unit
Purchase Agreement, the former owner of Montana Power LLC was responsible
for refunds not yet returned to ratepayers related to its prior sale of
oil and gas properties. The amount not returned to ratepayers at
February 15, 2002 was $27.9 million for which the former owner of Montana
Power LLC provided cash at closing. Additionally, at closing, the former
owner of Montana Power LLC was required to provide a minimum cash level
resulting from the net proceeds of the generation assets sale completed
by the former owner of Montana Power LLC. The cash provided at closing
amounted to $54.1 million. These proceeds represent the final gain to be
refunded to ratepayers plus funding for certain environmental
liabilities. Noncurrent regulatory liability has been adjusted to reflect
the net present value of the total refund to ratepayers totaling
$123 million which consists of the $60.0 million agreed to in the
stipulation plus the gain from the generation asset sale completed by the
former owner and regulatory assets have been adjusted to record the
approved stipulation.
5