As others have said, there's a lot you can do cr500. Registered mail is mandatory for firearms I'm sure. Australia Post have an internal investigation process, so get the details from the dealer and request an interview with APs security investigators. Remember there are mail delivery contractors in the loop these days, and AP staff will probably be just as keen as you to weed out any liabilities.

If all else fails, the dealer who sent the firearm should have some form of insurance cover for this eventuality.