Hi,
According to NSW legislation and Peter Gillard (registered engineer, you can find him on the RTA web site) the following formula is used:
3cc/kg – Naturally aspirated
2.5cc/kg – Forced induction
So if you in NSW get you rego papers out, find the tare weight, and calculate.
Also as mentioned above brakes, chassie strengthening, suspension etc will be required.
To sum it all up, Research & more research.
P.S. there is always exceptions to the rules.
Cheers.