check the vicroads website. they have a very long and boringly detailed legal document for you to read.
basically as long as its behind the drivers compartment you can do what you like, as long as you take out the seats, and mount it to a secure part of the car and not the middle of somewhere random...
BUT THIS DOES NOT MEAN YOU WONT GET DEFECTED. cops can defect you for whatever they want and its up to you to prove to them its legal.
-dan