this happens from time to time.  If Auspost, raise a query with them.  Expect it to take a few days at least to get a reply.

I've had a few customers raise this.  Including regular customers I trust.  This is not your responsibility though.  People steal mail or it gets misplaced (especially in apartments).  I've had an Auspost employee tell me that delivery van drivers have reported being tailed by people looking to steal parcels.  As a seller we can not take responsibility for lost or stolen items.  In apartments people often find missing mail by canvassing neighbours.  I recommend people use the parcel collect lockers.  

In my experience customers accept this, and Ebay will not force you to refund if you can demonstrate you sent with tracking.