EXHIBIT 5.1

BLAKE DAWSON WALDRON                                Level 32
                                                    Exchange Plaza
           LAWYERS                                  2 The Esplanade
                                                    Perth WA 6000
- ------------------------------
                                                    www.bdw.com

                                                    Tel + 61 8 9366 8000
                                                    Fax + 61 8 9366 8111

                                                    DX 169 Perth

                                                    PO Box 7438
                                                    Cloisters Square
                                                    Perth WA 6850
                                                    Australia

                                                    PARTNER
pSivida Limited                                     Roger Davies
Level 12, BGC Centre                                Telephone (08) 9366 8022
28 The Esplanade
PERTH  WA  6000                                     CONTACT
                                                    Alan Gibson
                                                    Telephone (08) 9366 8722

                                                    OUR REFERENCE

                                                    DRD:AJG:09 1395 3581

                                                    28 June 2006


Dear Sirs

REGISTRATION STATEMENT OF FORM F-3 (SUBORDINATED CONVERTIBLE NOTE)

We have acted as Australian legal advisers to pSivida Limited (COMPANY) in
connection with the Company's registration statement on Form F-3 (REGISTRATION
STATEMENT), originally filed on March 28, 2006 with the Securities and Exchange
Commission under the U.S. Securities Act of 1933 as amended (SECURITIES ACT).
The Registration Statement relates to the sale by the selling security holder
named in it of up to 57,631,650 ordinary shares of the Company represented by
5,763,165 American Depositary Shares that may be issued in connection with a
subordinated convertible note and warrant issued by the Company (ISSUE). We are
furnishing this opinion as exhibit 5.1 to the Registration Statement, subject to
the final paragraph of this opinion.

1.    DEFINITIONS

      In this opinion:

      (a)   ACN means Australian Company Number.

      (b)   ASIC means the Australian Securities and Investments Commission.

      (c)   ASX means Australian Stock Exchange Limited ACN 008 624 691.

      (d)   COMPANY means pSivida Limited, registered in Western Australia, ACN
            009 232 026.

      (e)   CONVERTIBLE NOTE means the subordinated convertible promissory note
            issued by the Company on 16 November 2005 and described in the
            section headed "The Offering" in the Registration Statement.

      (f)   CORPORATIONS ACT means the Corporations Act 2001 (Cth).

PERTH
SYDNEY
MELBOURNE
BRISBANE
CANBERRA
LONDON
PORT MORESBY
JAKARTA
SHANGHAI

BLAKE DAWSON WALDRON                                                28 June 2006

pSivida Limited                                                           Page 2


      (g)   RELEVANT JURISDICTION means the State of Western Australia,
            Australia.

      (h)   RELEVANT LAWS means the laws of the Relevant Jurisdiction and the
            federal laws of Australia as they apply in the Relevant
            Jurisdiction.

      (i)   SELLING SECURITY HOLDER has the same meaning as in the Registration
            Statement.

      (j)   WARRANT means the warrant to purchase American Depositary Shares
            issued by the Company on 16 November 2005 and described in the
            section headed "The Offering" in the Registration Statement.

      In this opinion, headings are for convenience only and do not affect
      interpretation and references to paragraphs are to paragraphs of this
      opinion.

2.    DOCUMENTS REVIEWED

      For the purposes of giving this opinion, we have examined the following
      documents:

      (a)   a search of the ASIC database in respect of the Company on 28 June
            2006 which shows that the Company is registered;

      (b)   the current constitution of the Company;

      (c)   minutes of the meetings of the board of directors of the Company
            held on 26 October 2005 and 14 November 2005;

      (d)   minutes of shareholders' meeting dated 15 November 2005; and

      (e)   the Registration Statement.

3.    SCOPE

      This opinion relates only to the Relevant Laws in force at 9.00 am
      (Western Australian time) on the date of this opinion.

      This opinion is given on the basis that it will be construed in accordance
      with the Relevant Laws.

4.    OPINION

      Subject to the assumptions and qualifications set out below, we are of the
      following opinion:

      (a)   the Company has been duly incorporated and is registered as a public
            company limited by shares under the Corporations Act;

      (b)   the ordinary shares underlying the American Depository Shares that
            may be issued:

BLAKE DAWSON WALDRON                                                28 June 2006

pSivida Limited                                                           Page 3


            (i)   on conversion of the Convertible Note;

            (ii)  as interest on the Convertible Note; and

            (iii) upon exercise of the Warrant

            will, if issued and paid for as contemplated in the Registration
            Statement, be legally issued and allotted and fully paid at the time
            of issue.

5.    ASSUMPTIONS

      For the purposes of this opinion, we have assumed that:

      (a)   if we have reviewed a draft of a document rather than a signed or
            executed copy, the document will be executed in the form of that
            draft;

      (b)   the minutes referred to in paragraphs 2(c) and 2(d) accurately
            reflect the business transacted at those meetings;

      (c)   no party has contravened or will contravene Chapter 2E (Related
            party transactions) of the Corporations Act or ASX Listing Rule 10
            (Transactions with persons in a position of influence) in connection
            with the Issue;

      (d)   no party has contravened or will contravene section 260A (Financial
            assistance for acquiring shares) of the Corporations Act in
            connection with the Issue;

      (e)   the Company was solvent at the time of and immediately after the
            Issue and is solvent at the date of this opinion;

      (f)   the meetings of the Company's board of directors and shareholders to
            approve the issue of the Convertible Note and the Warrant (and
            ordinary shares that may be issued in connection with the
            Convertible Note and the Warrant) were properly convened and:

            (i)   the resolutions set out in the minutes referred to in
                  paragraphs 2(c) and 2(d) above were properly passed as valid
                  decisions of the board of directors of the Company or the
                  Company's shareholders (as the case may be) and have not been
                  subsequently revoked, cancelled or varied; and

            (ii)  the directors of the Company have properly performed their
                  duties and all provisions relating to the declaration of
                  interest and voting were duly observed;

      (g)   where any obligation in connection with the Issue is to be performed
            in any jurisdiction other than the Relevant Jurisdiction, its
            performance will not be illegal or unenforceable under the laws of
            that jurisdiction;

BLAKE DAWSON WALDRON                                                28 June 2006

pSivida Limited                                                           Page 4


      We have not investigated whether the assumptions in this paragraph 5 are
      correct.

      None of the assumptions is limited by reference to any other assumption.

6.    QUALIFICATIONS

      Our opinion is subject to the following qualifications.

6.1   SEARCHES

      We have not made any independent investigations or searches, other than
      requests to ASIC for the company search referred to in paragraph 2(a) (the
      information disclosed in the search results rely on information lodged by
      the Company, and the search results may not be complete, accurate or up to
      date).

6.2   GENERAL QUALIFICATIONS

      (a)   We have relied, as to certain matters of fact, on information
            provided by officers of the Company.

      (b)   This opinion only relates to the laws of the Relevant Jurisdiction.
            We express no opinion on laws other than the Relevant Laws.

      None of the qualifications is limited by reference to any other
      qualification.

7.    CONSENT

      We hereby consent to the use of this opinion in, and the filing hereof, as
      an exhibit to the Registration Statement and to the reference to our firm
      under the heading "Legal Matters" and elsewhere in the prospectus included
      in the Registration Statement. In giving such consent, we do not thereby
      admit that we come within the category of persons whose consent is
      required under Section 7 of the U.S. Securities Act of 1933, as amended,
      or the Rules and Regulations of the Commission thereunder.

Yours faithfully

/s/ Blake Dawson Waldron