<PAGE>
                                                                    Exhibit 4.73

                     FIFTH AMENDMENT TO SECURITY AGREEMENT

     THIS FIFTH AMENDMENT to that certain Security Agreement, dated December 22,
1998, as amended pursuant to that certain Amendment to Security Agreement, dated
March 11, 1999, and further amended pursuant to that certain Second Amendment to
Security Agreement dated December 19, 2000 and further amended pursuant to that
certain Third Amendment to Security Agreement dated January 25, 2001, and
further amended pursuant to that certain Fourth Amendment to Security Agreement
dated March 6, 2001 (the "Security Agreement"), between Tipperary Corporation, a
Texas corporation ("Debtor"), and Slough Estates USA Inc., a Delaware
corporation ("Secured Party"), is made this 20th day of August, 2001, between
Debtor and Secured Party.

     WHEREAS, the parties entered into the Security Agreement to secure
performance and payment of certain Promissory Notes, (i) the first, dated
December 22, 1998, payable to Secured Party by Debtor in the amount of
$5,500,000 (the "$5.5 Million Note") and (ii) the second, dated December 22,
1998, payable to Secured Party by Tipperary Oil & Gas (Australia) Pty Ltd. in
the amount of $6,000,000 (the "6.0 Million Note"); and

     WHEREAS, the $5.5 Million Note was surrendered in connection with
additional funds in the amount of $1.0 million loaned by Secured Party to
Debtor, and was replaced by a Promissory Note, dated March 11, 1999 (the "$6.5
Million Note"); and

     WHEREAS, the $6.5 Million Note was surrendered in connection with
additional funds in the amount of $1.0 million loaned by Secured Party to
Debtor, and was replaced by a Promissory Note dated December 19, 2000 (the "$7.5
Million Note"); and

     WHEREAS, the $7.5 Million Note was surrendered in connection with
additional funds in the amount of $500,000 loaned by Security Party to Debtor,
and was replaced by a Promissory Note dated January 25, 2001 (the "$8.0 Million
Note"); and

     WHEREAS, the $8.0 Million Note was surrendered in connection with
additional funds in the amount of $4,000,000 loaned by Security Party to Debtor,
and was replaced by a Promissory Note dated March 6, 2001 (the $12.0 Million
Note"); and

     WHEREAS, Debtor is executing a Promissory Note, of even date herewith, in
the principal amount of $15.0 million (the "$15.0 Million Note"), in
consideration of the cancellation of the $12.0 Million Note and receipt of
$3,000,000 loaned by Secured Party to Debtor; and

     WHEREAS, the parties desire that the Security Agreement and collateral
provided therein shall secure repayment of the $15.0 Million Note in the same
respects as the $12.0 Million Note;

     NOW THEREFORE, in consideration of the loan evidenced by the $15.0 Million
Note and the promises of the parties evidenced thereby and hereby:

     1.  The Security Agreement is hereby amended in that, as of, and after the
execution of the $15.0 Million Note, all references in the Security Agreement to
the "Promissory Notes" shall be deemed to be to the $15.0 Million Note (in place
of the $12.0 Million Note as therein provided before this Fifth Amendment) and
the
<PAGE>

$6.0 Million Note. All provisions in the Security Agreement shall otherwise
remain unchanged, and the Security Agreement shall remain in full force and
effect.

TIPPERARY CORPORATION                   SLOUGH ESTATES USA INC.


By:  /s/ David L. Bradshaw              By:  /s/ Randall W. Rohner
   ---------------------------------       -------------------------------------
   David L. Bradshaw, President and        Randall W. Rohner, Vice President and
   Chief Executive Officer                 Chief Financial Officer

                                      -2-