Hi maggies1,
I am pretty sure you need to have the duty free on you when you clear Custom/Immigration.....so that being the case you couldn't pick it up in Brissie given you'll clear in Sydney. If you are planning on getting grog, I'd wait till you get on the ship and buy it there. Cheap cheap cheap.
I am fairly sure the SYD and MEL duty free providers have a 'buy now, pick up on return' facility. So you should be ok as you are leaving and returning through BNE (assuming they do the same thing). The only thing that may trip you up is that you will have an orange D sticker on your boarding pass when you do the SYD-BNE leg.
I suggest finding the name of the BNE duty free provider and give them a call. Make sure you speak to a reasonably senior person who understands the intricacies of your situation.
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The last leg is SYD-BNE as the "domestic sector of international flight", so they would be passing through customs in BNE.However, if the OP cleared customs in SYD, I wouldn't imagine they could pick it up in BNE.
The last leg is SYD-BNE as the "domestic sector of international flight", so they would be passing through customs in BNE.