<PAGE>
 
                                                                     EXHIBIT 2.2
 
                             FIRST AMENDMENT TO
                         BUSINESS PURCHASE AGREEMENT
                                        


     THIS FIRST AMENDMENT, effective as of August 1, 1998, is made this 30th day
of September 1998, by and between Infact Pty Limited, as trustee of the Infact
Unit Trust ("INFACT"), and Tier Technologies (Australia) Pty Limited ("TIER").

     WHEREAS, Infact and Tier are parties to a certain Business Purchase
Agreement dated as of August 1, 1998 (the "BUSINESS PURCHASE AGREEMENT"); and

     WHEREAS, Infact and Tier desire to amend the Business Purchase Agreement.

     NOW THEREFORE, for the promises made herein and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged,
Infact and Tier agree as follows:

1.  Amendment of the Business Purchase Agreement.

    1.1.  The Business Purchase Agreement is hereby amended by deleting
          Section 3.6(b) in its entirety and substituting the following in
          lieu thereof:

            "(b) Release of 29.412% (representing Tony Barker's relative unit
            holdings in Seller) of the Tier Shares and Escrow Cash in
            accordance with clause 3.6(a) is subject to Tony Barker, one of
            the Key Employees, remaining employed by the Buyer in the
            Continuing Business for a period of eighteen months from the
            Purchase Date, unless his Key Employee Contract is terminated by
            the Buyer for reasons other than cause as described in clause
            14.2(a) of that Key Employee Contract."

    1.2.  The Business Purchase Agreement is hereby amended by deleting
          Section 3.7(b) in its entirety and substituting the following in
          lieu thereof:

            "(b) Release of 29.412% (representing Tony Barker's relative unit
            holdings in Seller) of the Tier Shares and Escrow Cash in
            accordance with clause 3.7(a) is subject to Tony Barker, one of
            the Key Employees, remaining employed by the Buyer in the
            Continuing Business for a period of eighteen months from the
            Purchase Date, unless his Key Employee Contract is terminated by
            the Buyer for reasons other than cause as described in clause
            14.2(a) of that Key Employee Contract."

    1.3.  The Business Purchase Agreement is hereby amended by deleting
          Section 3.8(c) in its entirety and substituting the following in
          lieu thereof:

            "(c) Release of 29.412% (representing Tony Barker's relative unit
            holdings in Seller) of the Tier Shares and Escrow Cash in
            accordance with clause 3.8(a) is subject to Tony Barker, one of
            the Key Employees, 
<PAGE>
 
            remaining employed by the Buyer in the Continuing Business for a
            period of eighteen months from the Purchase Date, unless his Key
            Employee Contract is terminated by the Buyer for reasons other
            than cause as described in clause 14.2(a) of that Key Employee
            Contract."

    1.4.  The Business Purchase Agreement is hereby amended by deleting
          Section 3.9(c) in its entirety and substituting the following in
          lieu thereof:

            "(c) Release of 29.412% (representing Tony Barker's relative unit
            holdings in Seller) of the Tier Shares and Escrow Cash in
            accordance with clause 3.6(a) is subject to Tony Barker, one of
            the Key Employees, remaining employed by the Buyer in the
            Continuing Business for a period of eighteen months from the
            Purchase Date, unless his Key Employee Contract is terminated by
            the Buyer for reasons other than cause as described in clause
            14.2(a) of that Key Employee Contract."

2.  Business Purchase Agreement Remains in Effect.

    Except as specifically provided herein, all of the terms and condition of
    the Business Purchase Agreement shall remain in full force and effect.

3.  Counterparts.

    This First Amendment may be signed in any number of counterparts with the
    same effect as if the signatures were upon the same instrument.



                          [signature page follows]
<PAGE>
 
IN WITNESS WHEREOF, the parties have caused this First Amendment to be executed
by their duly authorized officers effective as of the day and year first written
above.



INFACT PTY LIMITED,
as trustee of the Infact Unit Trust:


           /s/ M. Van De Wiel                       /s/  A. S. Barker
   -------------------------------------   -------------------------------------
   Witness                                 Representative

               M. Van De Wiel                            A. S. Barker
   -------------------------------------   -------------------------------------
   Name (please print)                     Name (please print)



TIER TECHNOLOGIES (AUSTRALIA) PTY LIMITED:


          /s/ George K. Ross                     /s/ James L. Bildner
   -------------------------------------   -------------------------------------
   Secretary/Director                      Director

              George K. Ross                         James L. Bildner
   -------------------------------------   -------------------------------------
   Name (please print)                     Name (please print)