I think something is very, very amiss with Australia Post just stating it was delivered. 

Did the seller give you an investigation number ? Did the seller give their authorisation for you to make enquiries regarding the matter.

Lololol sorry if it sounds like the third degree

Australia Post investigations are a little more comprehensive than looking at the tracking details, and at this time of year when they are so busy I am very surprised that they would have conducted and completed an investigation in a couple of days. 

Perhaps post this situation on the AP facebook page, and see if you get more than a cut and paste response from them.