Have a look at these guys:
Cheap Car Hire, Compare Car Rental | Avis, Budget, Europcar, Hertz, Thrifty | Brisbane, Sydney, Melbourne Airport
Also look at picking the car up in town instead of at the airport. You can drop off at the airport for a nominal charge when you leave, but by picking up at a city office you avoid the premium location surcharge which over a long rental can be considerable.