Suzie sorry you're having to deal with this.
I'm in Australia too and I usually sell about one thing a month on ebay/bonz and so far have never sold to anyone in Australia - I'm usually shipping to the US.
I no longer use Registered Post but Express Post International instead - faster and more tracking. For a very valuable item I'd always use the Express Courier International service . . . but that's my suggestion for future shipping.
A couple of years ago I sent a $600 item Registered Post to a buyer in Canada. She contacted me after 2 weeks saying she hadn't received it although it looked to me that it had been delivered and signed for. I contacted Australia Post and they were very helpful. They got back to me with a link that showed me an image of the recipient's signature (a man with the same last name as my buyer). I advised the buyer and that was the last I heard from her.
I know your situation has gone further than this but perhaps Australia Post can assist from this end to help get things sorted out in your favour