Just googled - Importing yachts to Aust. ACS - and it gives a figure of 5% duty and 10% GST as a basic rule. One could deduct the cost of sailing it back here - within reason - and it does not mention fumigation or any other added costs.
I think one needs an import permit. Few years ago l imported a motorbike and l had to have one.
If you are serious about sailing it back, if you need a hand, l would be happy to help if you pay my expenses fully.
Before we get to that, whoever, l would fly over and inspect the yacht personally, out of water, if l was the buyer!