There are two methods of accounting for GST – cash and accrual.
Accounting for GST on a cash basis means you account for GST in the period that you receive the money or make the payment. You can only use the cash accounting method if you are a small business entity earning an aggregated turnover of less than $10 million, are not carrying on a business but your turnover is $2 million or less, account for income tax on a cash basis and/or you run a kind of enterprise that the ATO has agreed can account for GST on a cash basis regardless of turnover.
The alternative accounting method, accrual accounting, means that you account for GST during the period that you issued the invoice or were billed, regardless of whether payment has been received or made.