Most of the backpackers rent a motorhome to cross the country, or sometimes buy a used vehicule and resell it when they leave Australia. To rent your motorhome or a car in Australia, contact Apollo Camper! http://www.apollocamper.com/.
If you don't have the money to buy or rent a motorhome, you have the possibility to take the coaches. The Australian company Greyhound offers "passes" to make several trips within the country. Their buses are air-conditioned and comfortable.