Here's some reading:
https://www.infrastructure.gov.au/roads/motor/design/index.aspx
If you click on 'second edition' and 'third edition' you can see the different Australian Design Rules that came into play.
If your car is pre-adr, its means your car doesn't have to comply to these design rules. such rules include seat belts, reversing lights, door latches/hinges, turn signals, brake hoses, safety glass, instrument panels, fuel systems, noise, emissions etc.
However, certain modifications can put your car up to the brunt of 2013 ADR compliance, such as making a swb buggy (classed as ICV) meaning even the engine has to comply with todays sound/emissions), or making a lwb buggy (ie: rebodied vehicle) where the body needs to comply to todays standards with intrusion bars/lights/seat belts etc.