After a bit more research it looks like it's manufacture date, not registration in NZ date.
"2.1(2) A motor vehicle does not have to be fitted with seatbelts if it was manufactured, or first registered in any country, before 1 January 1955, but if seatbelts are fitted, they must be attached to seatbelt anchorages that are of an appropriate type, as specified in 2.6, and:"
https://www.nzta.govt.nz/resources/rules/seatbelts-and-seatbelt-anchorages-2002/#211
And also here on virms even though it says "First registered in NZ", on the tables themselves it reads either "registered anywhere" or "manufactured". So a bit confusing to read but the legislation itself looks fairly clear.
https://vehicleinspection.nzta.govt.nz/virms/in-service-wof-and-cof/general/vehicle-interior/seatbelts-and-seatbelt-anchorages
Happy enough to be wrong, but good to have the right info for anyone else looking.