The rules are set out in full on the NZTA website, but here are the main points as they apply to Class MA, MB, MC and NA vehicles. As a general guideline, most passenger cars are Class MA. Most passenger vans are Class MB, most 4WDs are Class MC and most vans and utes (primarily designed for carrying goods, not passengers) are Class NA.
Since October 2003 it’s been illegal to fit a bull bar to class MB and MC vehicles manufactured after that date if the bar adversely affects the performance of any frontal impact protection, which includes airbags and crumple zones.
However last year this rule was amended (32006/3)  for MC Class (wagon style) 4WDs, whose criteria is now the same as for NA Class (ute style) 4WDs.
The amendment states that vehicles of MA, MB and MC class that have a gross vehicle mass exceeding 2,500 kg no longer have to meet an Approved Vehicle Standard, although they still need to comply with the General Safety Standards, which means that the bull bar must be airbag compatible.
The General Safety Standards require a declaration sticker stating that fitment of the bar will not effect the vehicle’s compliance with the operation of the airbags – as has always been the case with utes.

The net result of the amendment is as follows:

- Wagon style 4WD (MC Class) vehicles exceeding 2,500 kg gross vehicle mass (GVM) may be fitted with bull bars providing they comply with the General Safety Standards and the correct declaration sticker is attached to the bar in a position readily visible to vehicle inspectors.

- Wagon style 4WD (MC Class) vehicles less than 2,500kg GVM and manufactured after October 1 2003 have to meet an approved vehicle standard, so under the criteria required bull bars may not be fitted to these vehicles unless full barrier crash testing has been carried out.

- Ute style 4WD (NA Class) vehicles of any GVM are able to be fitted with bull bars that comply with the General Safety Standards and have the correct declaration sticker fitted.
     

Typically what you will find is that the mid-sized wagons such as Prados, Pajeros, Pathfinders, Discoverys and KJ Cherokees  all exceed 2,500 kg GVM and can be fitted with bars and these will generally will become the cut-off point.

Some of the Wranglers are under 2,500 kg as are Vitaras, RAV4s and so on, and so won’t be allowed bull bars.

We must stress, however, that only approved airbag compatible bull bars may be fitted under this rule amendment – which means bull bars made by recognised manufacturers able to produce the necessary documentation.
In addition warrant of fitness inspectors have guidelines on bul lbar designs (in their Vehicle Inspection Requirements Manual) that are based on the External Projections Rule. An inspector may refuse to issue a warrant of fitness if, in their opinion, a bullbar:
• Slopes forward (unless the shape is necessary to fit closely to the vehicle's structure)
• Has sharp corners or edges
• Is more than 100 mm wider than the vehicle on either side
• Was attached as an after-market add-on to the vehicle's chassis or body by welding
• Has an unnecessarily wide gap between the bullbar and the shape of the vehicle
• Is excessively strong and rigid given the purpose it was fitted for.