And on any flight into Oz from Bangkok there is an additional security check at the entrance to the gate. I have seen people lose bottles of booze that they have purchased at duty free on numerous occasions.
It has nothing to do with the normal security scans that everyone must go through before passport control.
Every bag/ backpack etc is inspected by hand.
Duty free booze can be purchased, but must be in a sealed bag and delivered to the gate after the bag search. Unfortunately not every knows this and can lose a couple of hundred bucks of fine booze.