anything that alters the emissions of the engine is generally illegal and it is up to their interpretation of this as to what they ping you on, eg you'll get away with intercoolers etc but as soon as you up the boost, change turbo chargers, injection systems etc you need a full spectrum emissions test
you can call up regency and they will tell you everything, or do an application to modify vehicle and they will tell you exactly what needs to be done, most of it is hidden on the transport sa website but you have to dig around to find it.
edit:
a lot of them are listed here
http://www.vicroads....300/0/VSI22.pdf
and all the info should be able to be found here
http://www.infrastru...adr_online.aspx
if they are ADR approved or came on a different model of the same car then they don't need to be