Under New Zealand law, all children under seven years of age must use an approved child restraint appropriate for their age and size.

Children aged seven must be secured in a restraint if one is available in the vehicle.

 

see  details

https://www.nzta.govt.nz/safety/vehicle-safety/safety-belts-and-restraints/child-restraints/using-child-restraints-in-new-zealand/