Manufacturer tow ratings aren't enforceable:
https://www.nzta.govt.nz/resources/glovebox-guide-safe-loading-towing/guide-safe-loading-towing.html#howmuch
"Most vehicles have tow ratings given to them by the manufacturer specifying the gross trailer weight braked, unbraked, or both, that the vehicle can safely tow. Although the law does not require these tow ratings to be followed, the NZ Transport Agency recommends that they be taken into account."
The only enforceable law for lightweight towing vehicle I know of is this:
"the law requires that every light vehicle and trailer combination must be capable of stopping within a distance of seven metres from a speed of 30km/h."
Further reading:
https://www.nzta.govt.nz/resources/roadcode/about-your-vehicle/light-trailer-requirements/
Edit: Insurance may open a different can or worms