G'day Ross,
The new laws concern any vehicle that has been modified post 1989. (I think that is the correct date), and one has to prove that any modifications were performed prior to 1989, again, I cannot confirm the date.
No matter what the modifications are, and the year of the vehicle.
Someone with interests in either the standard vehicle industry, or the performance parts industry has lobbied the government and have enabled the laws to be passed.
In effect, they are saying that any modifications performed may have been done using parts that do not conform, or have not been passed to meet the Australian Design Rules, and don't have a current Australian Engineering Report, and therefore won't pass Roadworthy Certificates.
In the Hot Rodding Industry, a lot of parts that have been imported from USA have failed the ADR's certification, and what might be okay in USA, doesn't mean that it is automatically certified in Australia.
US DOT approved tyres, especially some of the Street Drag Tyres won't pass here, like their Seat Belts.
Bruce. >:D