Duty-Free shops are placed in such a way that travelers must leave the country of origin (sale). Australia is no exception.
Duty-free shops (or stores) are retail outlets that are exempt from the payment of certain local or national taxes and duties, on the requirement that the goods sold will be sold to travelers who will take them out of the country. Which products can be sold duty-free vary by jurisdiction, as well as how they can be sold, and the process of calculating the duty or refunding the duty component. - Duty-free shop
J. Caron makes this statement about the Sydney airport:
Don’t remember the exact layout in Sydney, but in most airports (if not all), you can’t get to the duty-free shops without going through immigration (where applicable) and security, which you generally won’t be able to do without a boarding pass for a flight.
The shop will also often require to see the boarding pass, and in some places will make a distinction between people travelling domestic or international. In some countries like the US, as there is not necessarily a separation between domestic and international travellers, and you can exit the departures area at will, they will not give you the goods right away, but deliver them during the actual aircraft boarding.
In some airports (especially in the EU), duty-free shops will actually sell duty-free when they can (you are actually leaving the country, or in the case of the EU, leave the EU customs union), and make a discount equivalent to duty-free for other cases.
I have seen duty-free sales of liquor and other items sold on in-flight only on some international flights.