Not sure to be honest, I've never had to show them the item. The interaction usually goes like this
Them: What are you declaring?
Me: A camera that I bought duty free when leaving Australia
Them: What's the value of it?
Me: It was $2600
Them: OK that's fine, just go down Lane X (whatever the lane is to just exit)
In one instance they got a supervisor since they didn't know what to do, and their supervisor just waived me through.
Multiple times I've seen people lined up at the duty collection counter whilst being waived through, so I don't know what triggers them to actually collect the duty. I do know sometimes people claim very expensive jewellery / watches etc. at the TRS counter, so maybe it's for that.
I suppose even if I paid duty, I'd still save ~$81 in GST since the first $900 should still be duty-free.
Sometimes too, they don't say that the lane they point down to, is the green lane, which is "nothing to declare", or the lane where the guy or girl just scan reads your IPC, and takes off the new fangled flimsy mug shot receipt off you.
They just point that way, down the metal and fabric rope lead, and they don't say anything, gruff as some of them are, ...
Left, right, left, right, turn here, turn down that corner, and then, find out where the point you or me to, in this case, is the "green" lane.
I don't buy duty free, and try not to interact with the poor sales girls trying to earn a commission, but still, I have no need for the drinks/knick knacks/other items, but always go down the way with tripidation.
Most of the time, coming out of the int security processing zone at MEL, am pleasantly surprised!
Green lane, whoopee.