I don't use RK anymore because of their non-existent customer service when something goes wrong, but in that instance, that's the postal service at fault, not RK. The mail service will use any plane available (on the airlines they have contracts with) to fly freight, but only if there is space available (to cost less to the postal service - reserved space would be for more expensive freight options). If there was a full flight on the direct passenger plane to Perth, then it would go to Perth via another destination if it is a cheaper option. All they (foreign postal services) care about it getting the packages to Australia at the cheapest means possible.... then it is the problem of Australia post to deal with it.

Now if RK had charged and promised a shipping option that guaranteed delivering within a set period of time, then they would be paying the postal service to pay airlines and Australia Post to prioritise that package (over regular/economy packages).