My suggestion : take a train instead of driving or do a combo.
Cheaper.
http://www.gsr.com.au/
If you take a plane to Perth - rent a car for a few days - drive
around to Port hedlands, Broome and return your car. Then you go by
train to South Australia - you will ride across the longest
straightest track in the world in Nullabour Plains ...from perth
into South Australia.
At South Australia - have a gander into Barossa valley for wine and
the germanish town in Adelaide Hills
Then return to the city , catch a country link train (Cheaper) ,
the GSR train (think got more views) or coach to Melbourne (free
views but butt hurt). For your reference:
http://www.countrylink.info/travel_passes/travelpass_agencies
Melbourne - rent a car again - wander about the city , go to
Grampians if it is not burning in summer as usual , Geelong ,
Ballarat , Bendigo . Return car
Take a coach to Albury Wodonga and then catch the train into
Canberra or Sydney straight. Canberra - stay for 2 days enough to
see the sights . Then proceed to Sydney. From Sydney , fly to
Adelaide , bypassing Queensland and take the Ghan GSR train to
Central Australia.
From Central Australia - go to alice springs , check out Ayer's
rock and then at Darwin - catch a plane to either Cairns or
Brisbane to see queensland.
Flights are cheap sometimes if you book in advance : we got virgin
blue , qantas domestic, jetstar and tiger airways now
You need about 1 month to do all these. That is what I planned for
my friend who is coming down from America.
GSR Train routes:
http://www.gsr.com.au/our-trains/train-routes.php