You can’t road register a quad bike or side by side in NZ even if you put all the lights/indicators/seat belts etc that are legally required on it, so I presume there is a mechanical reason why you can’t make a quad/side by side road legal.
NZ Post has recently started using glorified electric quad bikes for mail and small parcel deliveries which are fully road legal and can drive in the traffic.
What do the NZ Post quads have that other ones don’t allows them to be road legal?
My brief look suggests the NZPO vehicles are specifically designed to avoid being described as “ATV”s.
So, small wheels, road rubber, and 2wd would seem to be the go.