If the buyer was logged into ebay.com when they bought then they are covered by ebay.com buyer protection. This requires proof of delivery, which you have, and proof of delivery with signature if item plus postage was US$250 or more.  

I would send them the link showing the item was delivered, and ask them to check whether housemates or neighbours have picked up the item. Ask them whether they would like you to commence an investigation with Australia Post, and, if they would, phone AP on 13 1318 and see what they say.