It sounds like Australia Post have prevented anyone ever being able to force them to track down a missing item, by automatically listing items as delivered, even if it wasn't, or it was negligently sent to the wrong address... because they know that a sender or sending postal service can't dispute a "delivered" entry, and therefore can't force Australia Post to track down a lost item.
They're not accepting any responsibility for the items they carry, by preventing anyone from filing a claim against them, with "delivered" being used whether it was delivered or not to the right address.
