Actually you can still buy your duty free from any duty free shop outside the airport,but you need to,or not if you are brave enough lol,pack the fluid(booze or large perfume etc) inside your luggage and hand in the docket once you are through immigration,and yes you certainly can buy a lot cheaper than at the airport as they keep their price high because of high rental prices there plus they can because once there you have no other choice.By the way there is only one duty free in perth airport coming or going,just that they have recently changed name but same company.
Ted