<PAGE>
                                                                    Exhibit 10.6

[AGREED FORM DOCUMENT]



                                    FORM OF


                               DEED OF ADHERENCE

DEED is made on                             , 2004

BETWEEN

(1) SOUTH EAST WATER LLC whose principal executive office is at 600 Fifth
Avenue, 21st floor, New York, NY 10020 USA (the COVENANTOR);

(2) THE PARTIES whose names and addresses are set out in Schedule 1 to this Deed
(the SHAREHOLDERS);

(3) MACQUARIE INVESTMENT MANAGEMENT (UK) LIMITED incorporated under the laws of
England whose registered office is at 1 Ropermaker Street London EC2Y 9HD United
Kingdom (the ADVISER);

(4) MACQUARIE LUXEMBOURG WATER S.A.R.L. a company incorporated under the laws of
Luxembourg, whose registered office is at 5, rue guillaume Kroll, -1882
Luxembourg, Grand-Duchy of Luxembourg (the COMPANY); and

(5) MACQUARIE INFRASTRUCTURE COMPANY LLC whose principal executive office is at
600 Fifth Avenue, 21st floor, New York, NY 10020 USA (the GUARANTOR)

WHEREAS:


(A)   On 30 April, 2004 the Original Investors (defined in Schedule 1 hereto),
      the Adviser and the Company entered into a shareholders' agreement
      governing their relationship as shareholders in the Company and
      establishing the manner in which the affairs of the Company would be
      conducted (the SHAREHOLDERS' AGREEMENT).



      On 3 November 2004 Macquarie Global Infrastructure Fund 2 S.A. subscribed
      for 74 shares in the Company and on 4 November 2004 Macquarie Global
      Infrastructure Fund 2 S.A. subscribed for the Euro equivalent of Pound
      Sterling 4,145,525 (less Euro 1,868.50) A Preferred Equity Certificates
      and Pound Sterling 4,107,000 of B Preferred Equity Certificates in the
      Company pursuant to which it signed a deed of adherence on 4 November 2004
      agreeing to become a party to and to be bound by, the Shareholders'
      Agreement.


(B)   The Covenantor wishes to become a party to the Shareholders' Agreement
      immediately upon acquiring certain Securities in the Company and wishes to
      amend the Shareholders' Agreement with the effect that the Covenantor
      becomes a party thereto and, subject to the provisions of this Deed,
      assumes the rights and obligations of a Shareholder under the
      Shareholders' Agreement.

(C)   In order to facilitate the Covenantor becoming a shareholder in the
      Company, the parties have agreed to certain amendments to the Shareholders
      Agreement as set out in clause 3 below.

(D)   The Covenantor is a member of the Guarantor's Group and the Guarantor has
      agreed to guarantee the obligations of the Covenantor under the
      Shareholders' Agreement.

NOW THIS DEED WITNESSES as follows:

CONFIDENTIAL TREATMENT REQUESTED BY MACQUARIE INFRASTRUCTURE COMPANY TRUST AND
MACQUARIE INFRASTRUCTURE COMPANY LLC

                                                                          Page I
<PAGE>
[AGREED FORM DOCUMENT]


INTERPRETATION

1.    Words and expressions defined in the Shareholders' Agreement shall, unless
      the context otherwise requires, have the same meanings when used in this
      Deed.

ADHERENCE

2.    Subject to the provisions of clause 3 below, the Covenantor hereby
      covenants to and undertakes with each of the Shareholders, the Adviser and
      the Company and with each such other person who may from time to time
      expressly adhere to the Shareholders' Agreement (by way of execution of a
      deed or by way of novation) to be bound by and comply in all respects with
      the Shareholders' Agreement and to assume the benefits of the
      Shareholders' Agreement, as if the Covenantor had executed the
      Shareholders' Agreement as an original party thereto and was named therein
      as a Coinvestor.

3.    The parties hereby acknowledge and agree to the amendment of the
      Shareholders Agreement as follows, such amendments to be effective as of
      the date hereof:

3.1   The following change being made to clause 1.1:

      The insertion of a new definition as follows "SEW GROUP" means the Company
      and all of its subsidiary undertakings".

3.2   The following change being made to clause 6:

      The deletion of the word "Each" in the first line and the insertion of the
      words "Upon the request of the Adviser each" in its place.

3.3   The addition of the following clause as a new clause 8.5:

      "PROVISION OF FINANCIAL INFORMATION

      8.5   The Company shall provide the following information to the
            Shareholders:

      (a)   if a Shareholder requests for any specific month, within 30 days of
            the end of that month, management reporting in relation to the
            financial and operational performance of Macquarie Water (UK)
            Limited and its subsidiaries for the respective month; and

      (b)   a quarterly report in relation to the financial performance of
            Macquarie Water (UK) Limited and its subsidiaries, which shall
            include a discounted cash flow valuation; and

      (c)   consolidated audited annual accounts for the Company under local
            GAAP as soon as reasonably practicable but in any event within 3
            months of the end of each Financial Year; and

      (d)   if during any quarter the Company has any assets or liabilities
            other than its ownership of debt and equity in Macquarie Water (UK)

CONFIDENTIAL TREATMENT REQUESTED BY MACQUARIE INFRASTRUCTURE COMPANY TRUST AND
MACQUARIE INFRASTRUCTURE COMPANY LLC

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[AGREED FORM DOCUMENT]


            Limited, a quarterly report for that quarter in relation to the
            financial performance of the Company and its subsidiaries, which
            shall include a discounted cash flow valuation."

3.4   The addition of the following clause as a new clause 8.6:

      "MATERIAL CHANGE TO SEW GROUP

      8.6   Upon the authorisation or approval by the Board of the Company (or
            any committee thereof) or by the board (or any committee thereof) of
            any member of the SEW Group, of a course of action which may, or may
            be likely to, result in a material change to (i) the legal structure
            of the SEW Group and/or (ii) any intra-group financing and
            investment arrangements of the SEW Group including, without
            limitation, (a) any change in the balances (excluding the
            capitalisation of interest) or terms of any loans between Macquarie
            Water (UK) Limited and any entity that is a direct or indirect
            subsidiary of Macquarie Water (UK) Limited; (b) the declaration or
            payment of dividends by any direct subsidiary of Macquarie Water
            (UK) Limited in excess of the cash distribution associated with such
            dividend and (c) any material change in the balances (excluding the
            capitalisation of interest) or terms of any loans between members of
            the SEW Group (a MATERIAL CHANGE) the Company shall immediately
            notify, or in the case of authorisation or approval by any member of
            the SEW Group the Company shall procure that such subsidiary
            immediately notifies, all Shareholders in writing of the full
            details of such Material Change (the MATERIAL CHANGE NOTICE), and
            the Company shall use its best endeavours to procure that such
            Material Change Notice is given not less than 45 days prior to the
            Material Change being implemented by the Company or any member of
            the SEW Group."

3.5   The following change being made to clause 20.1:

      The insertion of the words "For the avoidance of doubt this provision
      shall not be applicable so as to require prior approval for any
      information contained in a document prepared to satisfy any reporting
      obligations pursuant to any U.S. federal or state securities laws or
      regulations or any stock exchange requirements." at the end of the
      paragraph as a new sentence.

3.6   The following change being made to paragraph 3.1 of Schedule 3:

      In paragraph 3.1, fourth line, the insertion of the words "at market
      value" after the words "(the PROPOSED TRANSFEREE)".

RETRANSFER

4.    The Covenantor and the Guarantor hereby undertake to each other
      Shareholder and to the Company that, if the Covenantor ceases or proposes
      to cease to be a member of the Guarantor's Group the Covenantor will
      forthwith transfer all its interests in any securities in the Company and
      assign its rights and obligations under its Preferred Equity Certificate
      Agreement to the Guarantor or to another member of the Guarantor's Group.

CONFIDENTIAL TREATMENT REQUESTED BY MACQUARIE INFRASTRUCTURE COMPANY TRUST AND
MACQUARIE INFRASTRUCTURE COMPANY LLC

                                                                        Page III
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[AGREED FORM DOCUMENT]


GUARANTEE

5.    The Guarantor hereby irrevocably and unconditionally guarantees the
      performance by the Covenantor of all of its obligations under this Deed
      and the Shareholders' Agreement.

CONTINUING GUARANTEE

6.    The guarantee given in this Deed shall be continuing and shall extend to
      the performance in full of all obligations guaranteed hereunder,
      regardless of any intermediate payment or discharge in whole or in part or
      performance in part.

WAIVER OF DEFENCES

7.    The liabilities and obligations of the Guarantor shall remain in force
      notwithstanding any act, omission, neglect, event or matter whatsoever
      whether or not known to the Guarantor, the Covenantor, the Company, the
      Adviser or the other Shareholders (other than the full performance of all
      obligations guaranteed hereunder) and the foregoing shall apply, without
      limitation, in relation to:

      (a)   anything which would have discharged the Guarantor (wholly or in
            part) whether as surety, co-obligor or otherwise or which would have
            afforded the Guarantor any legal or equitable defence;

      (b)   any winding up, dissolution, reconstruction or reorganisation, legal
            limitation, disability, incapacity or lack of corporate power or
            authority or other circumstances of, or any change in the
            constitution or corporate identity or loss of corporate identity by,
            the Covenantor or any other person; and

      (c)   anything which renders the Covenantor's obligations invalid or
            unenforceable under the Shareholders' Agreement and any defence or
            counterclaim which the Covenantor may be able to assert against any
            of the other Shareholders, the Adviser or the Company.

NO IMPAIRMENT OF GUARANTOR'S OBLIGATIONS

8.    Without limiting clause 6, none of the liabilities or obligations of the
      Guarantor under this Deed shall be impaired by the Company and other
      Shareholders agreeing with the Covenantor to any amendment, variation,
      assignment, novation or departure (however substantial or material) of, to
      or from the Shareholders' Agreement so that any such amendment, variation,
      assignment, novation or departure (including any which may have been made
      before the signing of this Deed) shall, whatever its nature, be binding
      upon the Covenantor in all circumstances, notwithstanding that it may
      increase or otherwise affect the liability of the Guarantor.

9.    Without limiting clause 6, none of the liabilities or obligations of the
      Guarantor under this Deed shall be impaired by the Company, the Adviser
      and other Shareholders agreeing with the Covenantor any amendment,
      variation, assignment, novation or departure (however substantial or
      material) of, to or from any agreement so that any such amendment,
      variation, assignment, novation or departure (including any which may have
      been made before the

CONFIDENTIAL TREATMENT REQUESTED BY MACQUARIE INFRASTRUCTURE COMPANY TRUST AND
MACQUARIE INFRASTRUCTURE COMPANY LLC

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[AGREED FORM DOCUMENT]


      signing of this Deed) shall, whatever its nature, be binding upon the
      Guarantor in all circumstances, notwithstanding that it may increase or
      otherwise affect the liability of the Guarantor.

DEMANDS

10.   Demands under this Deed may be made, and the liabilities and obligations
      of the Guarantor under this Deed may be enforced, irrespective of whether
      any demands, steps or proceedings are being or have been made or taken
      against the Covenantor and/or any third party.

RIGHTS SEVERAL

11.   The rights of the Company, the Adviser and each Shareholder under this
      Deed shall be in all respects several and the failure of any one or more
      of them to perform obligations under any agreement shall in no way affect
      the rights of the others of them under or in connection with this Deed. It
      shall not be necessary for the Company, the Adviser or any other
      Shareholder to be joined as an additional party in any proceedings by a
      Shareholder to protect or enforce its rights and interests under this
      Deed.

INVALIDITY

12.   If any provision of this Deed becomes invalid, illegal or unenforceable in
      any respect under any law, the validity, legality and enforceability of
      the remaining provisions shall not in any way be affected or impaired.

CONTRACTS (RIGHTS OF THIRD PARTIES) ACT 1999

13.   No person who is not a party to this Deed shall have the right to enforce
      any of its terms pursuant to the Contracts (Rights of Third Parties) Act
      1999.

AGENT FOR SERVICE OF PROCESS

14.   The Covenantor shall, unless it is a company incorporated in England and
      Wales, at all times maintain an agent for service of process and any other
      documents in proceedings in England or any other proceedings in connection
      with this Deed. Such agent shall be Macquarie Yorkshire Limited currently
      of Level 30, City Point, 1 Ropemaker Street, London EC2Y 9HD and any writ,
      judgment or other notice of legal process shall be sufficiently served on
      the Covenantor if delivered to such agent at its address for the time
      being. The Covenantor irrevocably undertakes not to revoke the authority
      of the above agent and if, for any reason, the Company requests the
      Covenantor to do so, the Covenantor shall promptly appoint another such
      agent with an address in England and so advise the Company. If following
      such request, the Covenantor fails to appoint another agent, the Company
      shall be entitled to appoint one on behalf of the Covenantor at the
      expense of the Covenantor.

NOTICES

15.   For the purpose of the Shareholders' Agreement the Covenantor's address
      for notices shall be as follows:

      Address: 600 Fifth Avenue, 21st floor, New York, NY 10020 USA

CONFIDENTIAL TREATMENT REQUESTED BY MACQUARIE INFRASTRUCTURE COMPANY TRUST AND
MACQUARIE INFRASTRUCTURE COMPANY LLC

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[AGREED FORM DOCUMENT]


      Fax No: 212 581 8037

      Addressed for the personal attention of: Peter Stokes

SUBMISSION TO JURISDICTION

16.   Each of the parties agrees that the Courts of England are to have
      exclusive jurisdiction to settle any disputes which may arise in
      connection with this Deed.

GOVERNING LAW

17.   This Deed shall be governed by and construed in accordance with English
      law without prejudice to the mandatory provisions of Luxembourg law.

IN WITNESS whereof this agreement has been executed as a Deed on the date first
above written.

Executed and delivered as a deed by            )
SOUTH EAST WATER LLC                           )
acting by:                                     )



Executed and delivered as a deed by            )
MACQUARIE INFRASTRUCTURE COMPANY LLC           )
acting by:                                     )


Executed and delivered as a deed by            )
[REDACTED PURSUANT TO CONFIDENTIAL             )
TREATMENT REQUEST]                             )
acting by:                                     )


Executed and delivered as a deed by            )
[REDACTED PURSUANT TO CONFIDENTIAL
 TREATMENT REQUEST]                            )
acting by:                                     )


Executed and delivered as a deed by            )
MEIF LUXEMBOURG HOLDINGS SA                    )



Executed and delivered as a deed by            )
MACQUARIE GLOBAL INFRASTRUCTURE FUND 2 S.A.    )



Executed and delivered as a deed by            )
MACQUARIE INVESTMENT MANAGEMENT (UK) LIMITED   )
acting by two directors /                      )
one director and the company secretary         )

CONFIDENTIAL TREATMENT REQUESTED BY MACQUARIE INFRASTRUCTURE COMPANY TRUST AND
MACQUARIE INFRASTRUCTURE COMPANY LLC

                                                                         Page VI
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[AGREED FORM DOCUMENT]




Executed and delivered as a deed by            )
MACQUARIE LUXEMBOURG WATER S.A.R.L.            )
acting by:                                     )

CONFIDENTIAL TREATMENT REQUESTED BY MACQUARIE INFRASTRUCTURE COMPANY TRUST AND
MACQUARIE INFRASTRUCTURE COMPANY LLC

                                                                        Page VII
<PAGE>
[AGREED FORM DOCUMENT]


                                   SCHEDULE 1

                                THE SHAREHOLDERS

<TABLE>
<CAPTION>
NAME                                                   ADDRESS
----                                                   -------
<S>                                                    <C>
[REDACTED PURSUANT TO CONFIDENTIAL TREATMENT REQUEST]  [REDACTED PURSUANT TO CONFIDENTIAL TREATMENT
                                                       REQUEST]

[REDACTED PURSUANT TO CONFIDENTIAL TREATMENT REQUEST]  [REDACTED PURSUANT TO CONFIDENTIAL TREATMENT
                                                       REQUEST]

MEIF Luxembourg Holdings S.A.                          a company organized and existing under the laws of
                                                       the Grand Duchy of Luxembourg, having its
                                                       registered office at 398, route d'Esch, L-1471
                                                       Luxembourg


Macquarie Global Infrastructure Fund 2 S.A.            a company organized and existing under the laws of
("MGIF" and together with the Original Investors,      the Grand Duchy of Luxembourg, having its
the "Shareholders")                                    registered office at 5, rue Guillaume Kroll-BP2501
                                                   L-1025 Luxembourg
</TABLE>


CONFIDENTIAL TREATMENT REQUESTED BY MACQUARIE INFRASTRUCTURE COMPANY TRUST AND
MACQUARIE INFRASTRUCTURE COMPANY LLC

                                                                       Page VIII