If the fault is not covered by the Australian Consumer Guarantee, then yes.
If the fault is covered by the Australian Consumer Guarantee, this is less clear as the ACCC states:
If faulty goods have to be returned to the place where they were bought, it is reasonable for the seller to pay appropriate freight costs.
It could be assumed that if Panasonic want you to send the product to a authorised service centre, then they should cover costs for freight where a fault falls under the Australian Consumer Law.
The alternative option is to see if you can return the product to Costco for them to manage. However, you may still up for freight and assessment costs if the fault is outside the ACL.
I would be surprised that you would need to send the microwave interstate as Panasonic should have service agents in capital cities and larger population centres.
