Best place to ask for information is the Motorsport NZ website and then go and have a look at the motorsport manual as every thing you need to know is there and it is current information.
Below is an extract that may assist you but you should read the complete section from the schedule before finalising your decisions:
4.7 Seating:
(1) Requirements: Seats shall:
(a) Be securely attached to the vehicle and adequately support the occupant(s) in competition, and
(b) Be installed in accordance with Part One Article 4.7(4), and
(c) Allow for the correct fitment/use of the Safety Harnesses.
(2) Seat Types: All of the following seat types are accepted under this Schedule:
(a) Original Seat: being a seat originally installed by the vehicle manufacturer to that particular vehicle model and type. All original seats shall comply:
(i) With the requirements of Part One Article 4.7(1)(a) and Article 4.7(1)(c).