When renting, watch out for:
1/. Excess fee in case of damage/accident (travel insurance often covers that, and I always use my travel insurance to cover that rather than paying an extra amount per day to reduce such amount)
2/. Excess mileage, many cheaper companies charge $20 a day but only give you like 100k free per day.
Make sure you have the collision damage wavier taken care of, many companies (especially smaller ones) will take the opportunity to stitch you where they can. Make sure you check vehicle thoroughly and record any dents, scratch marks or damage before you accept vehicle.
Everytime I need to hire I tend to find different deals on. Do your research on the net. You will be surprised how many different deals you can find. Try online agents too, like
www.vroomvroomvroom.com.au/ or www.Drivenow.com.au. They work like wotif or lastminute.com for hotels. Often you find surplus stock at reduced rate.
Big companies often have special deals too, also one way rental. Budget, Avis, Hertz, Thrifty. Abel is good too for $29 a day all inclusive, unlimited mileage if you rent for a week or more. Next to brand new cars.
You can also try the backpackers and rent a 'WICKED' van. Around $50 a day and you can live in it with a stove on the back.
Good luck and have fun!