The major sites when looking online for a car are..
www.carsales.com.au
www.carsguide.com.au
(allows you to sell your car for free)
www.carpoint.com.au
www.drive.com.au
Note: Expect to pay double the price for the same car in Oz, than what you would pay in the UK. This is because of high taxes.
On a sunny side though, petrol in oz is about 3x times cheaper than in the UK.