There are two main ways of transporting goods around Australia - Couriers or the Australia Post network - We use both.
If there is nobody in attendance to sign for the parcel when the delivery person arrives and "authority to leave" has been provided, they will simply leave the parcel unattended at the delivery address.
When "authority to leave" has not been given, then in the case of a courier they will take the parcel back to their depot to await your contact to arrange re-delivery. Re-deliveries can usually be arranged within 48hours but further information will be provided to you by the courier. In the case of Australia Post will take the parcel to what they consider to be your nearest post office, and you will need to collect it from there. Re-delivery is not a given option via Australia Post.
Either way, the delivery driver should leave a "Sorry we missed you" card which will explain what to do next.