we want to rent a campervan to travel australia, where is cheapest place to rent (4travelling) ?

Try Wicked Campers. They are great and cheap each one with an original mural on them. Lots of people will wave at you and introduce themselves to you just because you will stand out as a tourist. Enjoy your holiday.

Wicked campers - as above
Kea - keacampers.com
Travellers Autobarn - travellers-autobarn.com.au
Britz - britz.com.au
Only (as far as I recall) Traverllers Auotbarn hire 4wd campers. For the others you are not allowed off-road
Wicked campers allow small 'dinks' on the retrun free of charge.