Why?
The delivery guarantee is from Auspost, but their customer is the sender not yourself, even though you are the aggrieved party. Contact the sender for a refund as Auspost directed, because that is where your own ‘contract for delivery’ lies.
The sender should get their own refund from Auspost and that should not be made your problem.
Make sure Auspost includes your area in the guarantee, although a week is a bit much for any ‘express’ service and if it was not in the overnight coverage the item should neither have been accepted nor charged for express service.
