The receipt for posting + registered mail for a large letter shows the tracking number next to the addressee's postcode. Make a note of who that addressee is if your PO doesn'tt get you to fill out a registered mail form with the details on it.

What a shame AustPost don't give automatic tracking numbers to large letters -- instead, we have to pay $3.50 for registered mail to keep THEM from stealing the item we entrust them with.