It was mailed a long, long time ago -- 29-July -- and left Beijing for Australia on 04-Aug. Unfortunately, AusPost doesn't post tracking information for these items, so we can't know its location or if it was delivered. =/

You could inquire with AusPost to see if they have any idea as to the item's status. If that doesn't pan out, the sender can initiate a trace via China Post, but I'm not hopeful at this point in the game. =/

AusPost claims that inbound registered items aren't trackable in Australia, so I'm curious how they could know with certainty that it hasn't arrived.

I could be wrong, but I think that transit time to Australia is usually 2-3 weeks, which means that this one is likely to be lost. Only the sender of an item can open a trace, in which they contact China Post, who will then contact AusPost if necessary.

If this is something that you've purchased from a seller in China, you probably want to contact them to see how they want to handle the situation. Depending on how that works out, you may need to open an item not received dispute with eBay/PayPal/Aliexpress/etc. Frankly, until the item arrives at your doorstep, dealing with the postal service is the seller's problem.