National Code of Practice for Light Vehicle Construction and Modification (NCOP)
http://www.infrastructure.gov.au/roa.../vsb_ncop.aspx
This has a lot of very useful information for those interested. Its worth a look.
If you want to know what you can and cannot do. Most of it is here.
Please note that your state authority may have their own interpretation on what is acceptable and is also governed by engineering requirements particular to your modifications.