Registered mail can take longer than average because it must be signed for every step of the way even at the receiving end. Could be it is just hung up in customs in Australia. Perhaps your customer can check with a local post office to see if they can help track. Registered packages does have some kind of automatic insurance I think.
I've shipped lots to Australia and have never had a lost package though the occasional delay/fluke is inevitable.
Jen, the post office won't be of much help without tracking or insurance but you can ask your customer to check with any neighbors, family members who might have received the package (it happens!) or the local post office in case it is sitting there. It's been about 2-1/2 weeks - pretty long for domestic shipping but, well, anything can happen at any time. I give packages 30 days before considering it lost and refunding.