Have had that happen many a time. Usually l wait until the parcel comes back. Hopefully Aust post have put a sticker on the front with a tick on the box to say what the issue is (no such address, not collected, Left address, etc).

l usually message the buyer with the details and wait for a reply. If l don't get a reply after a week, l pull the buyer details and ring them.

Then one of two things will happen:

1. The buyer says 'opps, l forgot to change my address', or 'opps, l were away and forgot to collect'  l offer to resend once they have paid the postage again.

If the address is correct and Aust Post made a boo boo, l take the parcel back to the post office and postie writes 'please deliver, address correct'. No extra postage charge needed.

2. The buyer dosen't answer phone or message, has vanished off the face of the earth. l store the parcel in my stock room until which time l hear from the buyer. (l have one parcel in the stock room that has been sitting there for almost a year. Can't even refund as their paypal email addy is invalid).