Following on from that Youi insurance thread regarding the 'roadworthiness' of a vehicle I did a little research and I've come across a bit of information from the RMS (NSW) and from the Department of Infrastruction and Transport regarding modifications of vehicles.
Vehicle Standards Information Sheets
VSI 6 - A Guide to Light Vehicle Modification
VSCCS Bulletin 1 - List of Certified Engineers
VSCCS Bulletin 2 - Means to Demonstrate Compliance with ADR3
VSB 14 - National Code of Practice for Light Vehicle Construction and Modification (NCOP)
The VSI sheets are guidelines from the RMS regarding modifications to vehicles. VSI 6 will be the one to read as it specifically refers to modifications to light vehicles.
The VSB14 bulletin/NCOP contains the specifics in regards to what and how engines, chassis, brakes, body modifications etc will be certified by licensed certifiers.