Seems you have proof of lodgement because a scan event says received by Australia Post. So you should win the INR case no worried. That means the buyer will not get a refund. Now if they really have not received their item it's then up to you if/not you refund them. If you put a claim in with Australia Post they are only likely to refund the actual cost of postage not the value of the goods unless you purchased insurance.
The rules under Australian Consumer Law are that the buyer is deemed to have received their item when the seller lodges it with a carrier. It's all in the Sale of Goods Act.
So you ad the seller have protection from what I see here.