The registration you are referring to is Historic Registration.
To qualify for historic registration the vehicle must be in the same configuration it left the factory OR only with optional extras at the time. In other words, if you want to modify it, you WONT qualify for historic rego.
IF your car is 100% stock, the registration is approximately $70 per year, Plus any associated club fees. You must be a full financial member of a club that is authorised for historic rego, and can verify the historic status of the vehicle - for example, the Falcon GT Club of SA is able to have all models of Falcon GT and historic Falcons/Fairmonts under their club. Under historic rego it is a logbook system, that is, you MUST compete the log book for the expected journey before leaving your driveway, as if you dont fill it out BEFORE leaving your house, your car is UNREGISTERED and UN INSURED.
So its not as easy as cheap rego, as with club fees will often come in at around $120-150 per year, BUT its good alternative for a truely historic car (>30 years old, ie. pre 1978)