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.
Note also that in the event you decide to go to another country and buy alcohol duty free on your way back, you have to pay import taxes when you arrive, unless you have less than the allowances, which are usually on the order of a couple litres of hard alcohol, so you couldn’t buy « a bunch » of whisky and benefit from the regime.