National postage is fixed by weight, for the most part. Remote postcodes that are accessible only by barge or long haul trucks typically have their stuff come in through other logistical means, like in with the food delivery, or sometimes in the cargo hold of a visiting flying doctor.

It is probably a nightmare for those edge cases, but most Australians live in the major cities.

But then, theres places like the housing estate that I live in. Its a five minute drive to the local post office, yet people a few streets away dont get their mail delivered, and need a Post Office Box.

The local post office has no spare PO Boxes... as far as I am aware.. so uh... yeah.