Not exactly sure what you are trying to do, but the Aussie Airpass is specifically for the entire journey from the US to Australia, plus the 3 domestic flights (or more) within. Since you already have the US-OZ legs 'purchased', I don't really see the Airpass as a possibility.

Why not book the one-way flights through www.qantas.com.au or try a travel provider (like webjet.com.au) to book a multi-city trip within Australia? Qantas often price their segments one-way so you should be able to pick up a good deal - BEFORE you arrive in Oz.

Eg. sample itinerary is pricing out about $A500 below.
Thu 16 Nov 2006
From: Sydney To: Ayers Rock
Mon 20 Nov 2006
From: Ayers Rock
To: Melbourne (through Sydney)
Fri 24 Nov 2006
From: Melbourne
To: Sydney