The person brining the item back into the country must declare it on their inbound passenger customs declaration card as they are bringing into the country an item that was obtained overseas of a value greater than $900. The fact that it was a gift and purchased by someone else is irrelevant when it comes to bringing things into the country.
Whether or not someone is likely to be caught making a false customs declaration does not alter the fact that it is against the law to make a false customs declaration.
Unfortunately the law has no idea that the jewellery was obtained overseas. As far as customs is concerned the jewellery was a gift from someone in Australia while in Australia. I am not saying to lie deliberately but customs only needs to know what you feel they need to know.
Maybe I am naive and just asking for trouble but I bring back many things from overseas, some I have bought while others are gifts, and as I am not sure of the value they are under <$900 for the purposes of the customs declaration.
Does anyone seriously declare clothes, gifts or jewellery on a customs declaration? So if someone buys a $3,000 suit overseas they should note this on a customs declaration? In my opinion it is irrelevant that it may just be illegal but if someone was wearing this suit customs would have no idea it was purchased or gifted overseas unless it was declared on the form or customes were tipped off. They won't come up and say "geez that is a noice suit did ya buy it overseas on this trip?".
On my first trip overseas back in the 90's I came back with a $600 leather jacket, jewellery for myself and family worth well over $1,000, countless gifts from aunts, uncles, godmother, cousins, friends and souvenirs I bought myself etc. I would have absolutely no idea what these gifts were worth, but they were worth lots, but I was not going to declare these items so I can pay stamp duty in Australia after I had paid the relavant duty and VAT overseas. Sorry I am not that silly to pay through the nose twice.
Now if I got caught the leather jacket was purchased in Australia many years ago, and as far as I am aware there is no legal requirement to have a receipt for everything in your possession, the jewellery was taken overseas to be repaired as there is no 14 carat stuff in Australia and the gifts are worth very little.
And I understand this is a public forum and we should not be giving advice on what to do or what not to do when leaving or reentering Australia but on needs to make informed decisions wisely. The chances of getting caught out and customs being aware are very miniscule. Now there is a huge YMMV on this one and I can safely say that to date no one I know of has had any issues coming back into Australia with purchases or gifts.
P.S I have probably cursed myself and will be questioned by customs when I reenter Australian after my next trip to Greece with gifts from the relatives....