Not sure of what is required in NZ to get it out,,but it is not considered a firearm here (unless you live in NJ).
I bought an antique status Lee Enfield military rifle from NZ. It was shipped by Post Office and delivered to my door.
The shipper placed all the supporting documentation 'Joe in Charlotte' mentions, proving it was an 'antique' in with the customs forms and paperwork. Extra copys inside as well as the packet attached outside.
It made it here in about 1 week no problems.