i drove a rental at queensland, not a biggie.
unless you are going to inland, you really dont need a rental to travel Australia - most drives are boring from place to place and qantas / virgin blue always have some dirt cheap airfare on sale.
in big cities public transportation is more convienient than rental car, especially when you factor in the hassle to find parking space.
in queensland, places such as cairns, rent a car would give you a lot more mobility though there are buses going up and down the coast.