A New Tax System (Goods and Services Tax Transition) Act 1999
If, before 1 July 2000:
(a) any consideration is received in connection with a supply, or provided in connection with an acquisition, that you will make on or after that day; or
(b) an invoice is issued relating to a supply or acquisition that you will make on or after that day;
for the purposes of determining the tax period to which GST or input tax credits are attributable, the consideration is taken to have been received or provided, or invoice taken to have been issued, during your first tax period after that day.
Division 29 of the GST Act contains rules about attributing GST and input tax credits to tax periods.