NZ systems have the ability to enter 2 FF numbers in a booking. One for earning purposes and one for non-earning purposes. You'll need to call Air NZ and ask them to enter your Velocity number in the earning field and your NZ number in the non-earning field and this should make your NZ status show up on the boarding pass and pax manifest and NZ systems (allowing you lounge access, Space+, extra bag, special onboard treatment etc) while crediting the flights to Velocity.