everyone is alaways wondering what is legal or illegal to do, so this will help, and it also says what supposedly HAS to be done with an engeneering certificate or what is appropriate:
http://www.dotars.gov.au/transreg/vsb/vsb_ncop.aspx
it states some things you can fit yourselft, such as stereo's and extra speakers, interior mounted guages (check it for whats legible), some exterior lights (i think it says fog lights and driving lights only), alarms, and sign markings on cars, as long as they dont reflect light :S, and some other paint applications, just read through it if you have the time..
remeber its still a draft so far, but it will be put through soon as the final copy