That is partly correct Ross. The HRSCC scrutineer to MNZ regs. The VCC have their own regs and do not require seat belts or roll bars.
The crux of the "problem" is that when MNZ promulgate rules they do not seem to differentiate between forms of car racing that have relatively low levels of risk and those that have higher levels of risk. Most people would agree that a Standing 1/4 Mile Sprint is less dangerous than a field of F5000 cars racing on a circuit. But the regs often do not reflect this.
Maybe the VCC have some common sense. As Ray has quite rightly pointed out in Post #120 MNZ do not differentiate between different designs and/or different materials used in race car construction. VCC probably acknowledge that if you tried to fit a roll bar to say an MGTF the roll bar itself may be strong enough to prevent injury in the case of a crash but what about the attachments to the car? There are very few possible attachment points and they are weak so will likely fail in a crash. Though not required under VCC regs there are very few (if any cars) racing under VCC regs that do not have some form of roll protection or seat belts. They just don't get overly officious about "out of date" seat belts or roll bars that don't meet the same standards as those required for F5000 cars.