In NSW you can get any motor engineered into any car but it all comes down to the class of registration and class of vehicle.

To fit in the 'modified production vehicle' class the engine has to be under the before mentioned size by Kegs.
( this is the easiest class to engineer with the least criteria to meet)

Any bigger and the car will have to be classed as 'Individually constructed' and the vehicle will have to meet the Australian Design Rules of 2004.

It is posible to work with an engineer and to build the car to meet the Modified production class and still have a larger engine
But the cost and requirements would overcome the final outcome

You can get all your information at www.rta.nsw.gov.au
there you can get a list of engineering signatories to contact to see what is required for this conversion and if they will pass it.