One of the major expenses you'll encounter while in Oz is transportation. If you plan on covering a significant part of the country then I'd suggest giving how you'll get around some serious thought.

What I Did
I didn't give it much thought beyond my first month. I'd planned to arrive into Sydney and fly with Virgin Blue up to Townsville a week later before busing it to Cairns to really start my trip. From there I hitched a ride on a yacht down the east coast, rented mini-mokes on Magnetic Island, bought a Greyhound kilometre pass, flew from Sydney to Melbourne, bused up north, hitched around a bit with locals, bused back down to Melbourne, shared a car with friends across the Nullabor to Perth, shared another ride up the west coast with more friends, then bused my way back to Perth before flying out to the UK.

You can also buy a car of your own, take a train, rent campervans, travel on backpacker buses, or even take organised tours around Australia. I've complied as much information as I can about each mode of transportation including my personal experiences and opinions on the methods I used.

Australia is a bit of a logistical nightmare, so good planning will go a long way and knowing a bit about each method will open up more options for you once you get there!