If AusPost does not help with the investigation, you could contact the ombudsman.

Anyway, I remember a buyer a few months ago who posted here that he had bought alcohol and received an empty box... He seemed to be a reasonable, honest person, as he had contacted AusPost and had not opened a case and had also been polite in his interaction with the seller. Or maybe it is the seller who posted, I don't remember. 

It does seem to be a bit strange...