The rifle can only be taken while in Australia Post hands so that means a employee or contractor or did the gundealer actually send it? ask him for reciepts and the gundealer has to declare it is a firearm . I would look at the gundealer and the sending post office and contact the Australia Post State Manager and get him to look into it. The gundealer may be legally liable as he is legal last "owner" and if he can not prove he mailed it its on him
Steve