The following information is an extract from the Queensland Government Department of Transport and Main Roads:

Complex modifications such as engine upgrades, gearbox and rear axle changes, vehicle body modifications and steering and brake replacements are significant modifications to a vehicle and can seriously affect the safety of a vehicle if not performed properly.

Complex modifications to light vehicles must be certified by approved persons, providing the modifications have been performed in accordance with an approved code of practice.

The approved codes of practice are based on accepted vehicle engineering practices and the requirements of the Australian Design Rules for Motor Vehicle Safety. It is intended to supplement the recommendations of the original vehicle manufacturer in relation to vehicle modification techniques or standards. It is important to note that the Australian Design Rules and the original manufacturer’s modification guidelines take precedence over the codes of practice.

Before beginning to modify your vehicle check out this website:  http://www.tmr.qld.gov.au/Safety/Vehicle-standards-and-modifications/Vehicle-modifications