Before you do anything, take a deep breath...

Wait until Australia Post has finished their parcel investigation before you decide to do anything. 

If you have only emailed them, then ring them as they need all the details that you have regarding your parcel.

For example, if the buyer is claiming that parcel would not have fit into her mail box, they will want to know this, and how big the parcel was.  The sooner you do it the better, as they will question the courier who may remember the particulars of the delivery OR safe drop arrangements they may have with the buyer.

If AP thinks that the courier or the buyer or even the neighbours in that street, is up to no good based on a pattern of non-delivery in that area, they will manage that process.  Fortunatly, I have found that the investigation process to be relatively quick.

IMO stay in touch with your buyer so you can keep them in the loop with the process, you can chose to provide them with the number of investigation and authorise them to discuss the case with AP.  I think that it is better to do this, after all it is the goods they have paid for that are lost.