Okay so it won't let me edit my initial post, but here is what led to ebay deciding in my favour.

Firstly, upon informing ebay that the buyer sent back an empty box, I was asked to sign a stat dec affirming that I received an empty box.  They e-mailed it to me, and I had to get it witnessed by a JP, take a pic of it, and then upload it to a link in the e-mail.

Separately, I contacted Australia Post via live chat, gave the tracking number, explained the situation and asked if the item was weighed whilst in transit back to me.  They said yes, and so I gave the other tracking number and asked if it was weighed whilst in transit to the buyer.  

Long story short, there was a massive difference in weight between the item I sent, and the item the buyer returned, proving that the buyer either sent me an empty box, or something very very light - but not the original item.

Australia Post was nice enough to put this information along with the tracking numbers, in a tidy little e-mail to me.  I took a screenshot and uploaded it with the stat dec, and a couple hours later this nightmare was over.

I do not know for certain what role the Australia Post information played in ebay's decision, because they don't exactly debrief you.  Maybe they just wanted the statutory declaration and would have decided in my favour regardless when they received it.

But they explictly said that the buyer was not refunded, and they removed his feedback.  So some common sense won out.

I still haven't opened the box and I'm thinking about taking it to the police and seeing if they have any interest in pursuing the matter.  My gut says no, but it might be worth a try.  As previously mentioned, I don't want this buyer trying this scam with some other seller.

Thank you again everyone for your input.  It's nice to know there are people so invested in helping others.