http://www.ltsa.govt.nz/factsheets/44b.html
It is actually fairly easy to qualify for Special Interest Vehicle - it must meet three of these criteria;
* the vehicle (or its make, model and submodel) is identified as being a collector's item in one of the following magazines or their respective websites – Australian classic car, Car and driver (US), Automobile (US), Motor (Australia), Motor trend (US), New Zealand autocar, New Zealand classic car, Road and track (US), Top gear (UK), Top gear NZ, Unique cars (Australia) or Wheels (Australia)
* the vehicle's make and model has been (or was) manufactured in annual volumes of 20,000 units or less
* the vehicle is, and was manufactured as, a two-door coupe or a convertible
* the vehicle is, and was manufactured as, a high-performance vehicle.
As for limits - 200 in any one year, which is a lot considering you can't apply for a permit if you have already had one issued in the last two years. I don't know whether you can rego these vehicles, or if it just lets you import them (for racing or whatever). You could make the case from these criteria for just about all of the cars on that site.