The South Australian Security of Payment Act provides for interest at s13(2) of the Act as follows:

(2) Interest is payable on the unpaid amount of a progress payment that has become due and payable at the rate—

     (a)     prescribed under the Supreme Court Act 1935 in respect of judgment debts of the Supreme Court; or

    (b)     specified under the construction contract,

whichever is the greater.

South Australian practitioners should note that the Supreme Court Act 1935 can be found here at this link.  Always check however, as at March 2013 Section 261(3) states:

Part 19—Interest on judgment debt

261—Interest on judgment debt

(3)     with effect from and including 1 July 2010:

(a)     in respect of the period from 1 January to 30 June in any year or any part of that period in any year,  the cash rate of interest last set by the Reserve Bank of Australia prior to that 1 January, plus 6 percent;

and

(b)     in respect of the period from 1 July to 31 December in any year or any part of that period in any year, the cash rate of interest last set by the Reserve Bank of Australia prior to that 1 July, plus 6 percent.