And now the $64,000 dollar question, as the game show used to go. Unfortunately I cant say as I am in Sth Oz. I can tell you the subject is covered in Vehicle Standards bulletin 14 National Code of Practice for Light Vehicle Construction and Modification (VSB 14).
The copy I have is Version 2.0 -1 January 2011 and runs to 7 pages. In the Preface, P3/7 para 2 "The underlying principals governing this document are in accordance with the national transport reform process that seeks to reduce barriers to trade and inconvenience to both industry and the public in the operation and registration of vehicles across Australian State and Territory borders.
Now here is the prize question!!!!!
Para 6 reads " However its adoption as an integral part of the registration process is the individual responsibility of the Registration Authority of each jurisdiction.
In Notice to Users of VSB 14 page 5/7 you will find all the if and buts regarding, policy, acceptance legislation, changes modification etc etc.
Not sure any of that is a help, as I think you would have more going for you if you wanted a same sex marriage at the moment rather than equal treatment on rego issues between the states. Maybe we could have a national postal vote on the subject?