I am a seller, and on a couple of occasions a buyer has contacted me re a parcel not arriving.   I always negotiate .... contact Australia Post (they are always very helpful).   Also, I askthe buyer  if it's possible for them to check with their local PO.

I have never had a parcel LOST by Australia Post.   If there is a return address on the parcel, it should be returned.

My advice, talk to the sender  ... there may be a perfectly logical explanation.   (It's also possible there is a mistake in the address, and hopefully, the receiver is honest, and will return it)

Good luck !!