OK, in Australia all cars are built to comply with ADR's (Australian Design Rules)
http://www.infrastructure.gov.au/roads/motor/design/adr_online.aspx
Now that will boggle the mind.

now every company that builds a car so say for example Ford produce a car or import from another factory it must be shown to comply with the relevant ADR's. Pollution, safety requirements etc.
Now what I am doing and have done is put a a 1948/49 cab on a later Chassis. This chassis complied with the ADR's for 1977 and as the cab I have used did not comply with any of these ADR's and the Cummins engine complies with another set of ADR's I have to get a registered Engineer to write up a report to show how all the changes I have made comply.
Seats and seat belts for example have to comply as does windscreen and many other items. I suppose you can say the Engineer helps me prove my vehicle is safe and that all changes are not illegal.
Now as Rod said the different states here interept (read) the ADR's differently. State Govenrments vs Federal Governments....

and that is a tin of worms with the LPG issue....Sorry another topic.
Any way what I have done is modify a light truck and the engineer is now looking to see that all the work I have done complies with what is required. I have taken a 1948 Ford cab and updated it to comply with the ADR's that where required for the Chassis (1977) and the Cummins engine (1991) As I have reused many parts from the chassis like the steering column and brake booster, I can prove nothing has changed and that I still comply with the relevent ADR's. Example I have used the steering column that came with the chassis so it still has a steering lock to comply with ADR 25 -Anti theft locks.
All a lot of fun when you use new LED style lights that comply with the new ADR's so with my vehicle it has been interesting as I have complied with some ADR's that the chassis, engine and cab never had to comply with.
I hope that explains to you

Now if you want to build a Hot Rod that opens another tin of worms.
http://www.infrastructure.gov.au/roads/vehicle_regulation/bulletin/street_rod.aspx
or this one >
http://www.infrastructure.gov.au/roads/vehicle_regulation/bulletin/vsb_ncop.aspx