You might be battling to find a secluded beach, Phuket is heavily populated. I'd be inclined to reserve a secluded spot in a resort garden restaurant. Also there's a fair bit of rain about this time of year so that's to be considered too.
You'll see lots of companies around, especially in Phuket town and Patong beach, that specialize in shipping stuff back to Oz. Those guys are the pros and the ones to talk to. You can shop around and compare prices. Make sure you collect the stuff immediately after it's cleared in Perth or you'll have heavy storage fees to pay.
Gold shops are everywhere, once again check them out, have a look, see what your GF likes. Thai gold is very pure, soft bright yellow. Prices are sky high at the moment. I don't think there'll be much difference in the prices between gold shops, they're more or less standardised in prices.
If you want wine with your secluded dinner then bring a bott or 2 from Perth as wine is rather expensive here, often double or triple the prices in Perth.

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