Recent changes to personal importing rules means you can only ship a car to Oz once every 5 years after owning the car here for the last 12 months continuously unless it falls under a list of exotic cars e.g Fez, Lambo. Different cars will have a different margins compared to the same car in Oz. Karman is a good website to get more info on logistics of it all or check the below link.
http://www.moving-to-melbourne.co.uk...-to-australia/