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On a private farm in NZ are you allowed to have people sitting in the back of a ute?
The road rules we have access to are for vehicles used on public roads, the rules are not enforced for vehicle use on private land. On the road, the rules state that a person must not ride in or on a vehicle, or in or on an object conveyed on a vehicle, in a manner or position that may be liable to cause injury to that person.
The definition of a "road" is much broader than public roads; it includes other places "where the public have access to, whether as of right or not". That includes for example beaches and private campus roadways; road rules do apply there. I suspect though that it wouldn't extend to private areas on (say) a farm, esp. behind fenced/gated areas.