Here is an extract from the Australian Taxation Office web site:
"Exports of goods
Exported goods are GST-free if they are exported from Australia either before, or within, 60 days of the first of the following two events:
the supplier receives any payment for the goods, or
the supplier issues an invoice for the goods.
In the case of goods paid for by instalments the payment referred to is any part of the final instalment and the invoice referred to is the invoice for the final instalment.
The supplier can apply to the Tax Office to extend the 60-day period.
If you need help in applying this information to your own situation, contact us by phone.
International callers can contact the Tax Office by phoning +61 2 6216 1111 between the hours of 8.00am to 5.00pm (Australian Eastern Standard Time or daylight-saving time) and ask for your call to be transferred to the appropriate area within the Tax Office. "
Hope it works for you.