Try ABC National talk-back. Australia Post does not need the bad press given they lost $30k worth of artwork a few months back ... but then that was uninsurred

and fell off the back of the truck. You've done everything as required by law ... but a rifle is now missing with no concern shown by Police or Australia Post, let the ABC chase it up and your State and Federal members as well. Might as well make them do some work.
Also... some Post Offices will not handle anything over 1m length as it wont fit the steel cages they use for transport. It may very well be sitting in a Post Office somewhere, this is particularly so in the smaller Post Offices where long packages are placed to the side.
Good luck...