The buyer cannot ''open an investigation'' only the seller can report it to Aust Post as lost (not sure about stolen from the package).

I suggest that you do so asap so the refund can get underway, could take a few weeks.

More info here- http://www.ebay.com.au/gds/What-to-do-if-Australia-Post-loses-my-item-/10000000001162511/g.html

Good luck!