"As I said I would not have bid at all if I had been told it was not Australian stock" sounds exactly like a change-of-mind to me. 

I would message the buyer and say something like:

"I am so sorry that you have changed your mind about wanting this camera.  Return it to me and I will happily refund your payment in full"

The reason for this wording is to try and get a reply from them acknowledging that they have changed their mind about wanting the camera.  Then, if they lodge a PayPal dispute for SNAD then you can forward the messages to show they have changed their mind.  If they return it you refund them and offer to an underbidder.  I hope you kept a copy of the serial number of the camera in case they return a dodgy one of the same model with a much higher shutter count (has been known to happen on eBay).