Yes, while the Greens did not wish to order the Anzac frigates, the Labour prime minister of that time did wish to order new frigates to replace ageing Leander class frigates. While New Zealand could have purchased cheaper smaller frigates from the United Kingdom or elsewhere with the same weaponry, the smaller frigates would not have had the range of the Anzacs. Fortunately, New Zealand was able to receive enough offsets with the Anzac shipbuilding program, the balance of payments were equal. A very significant development for both nations which the British could not match.
However, in the very near future, the anti-submarine torpedoes and NATO Seasparrow anti-air missiles are reaching the end of their shelf life. New Zealand will have no choice but to upgrade these weapons systems.
I agree, a third frigate is necessary to sustain a deployed frigate commitment beyond six months at all times. While having two frigates means one will be available for a sudden deployment, there is no guarantee that New Zealand can continue to sustain a deployment for a long period of time.
I also agree Harpoon missiles are a defensive weapon. While the current government has to please the Greens to maintain office, I cannot see a National government ever linked with the Greens. There is a better chance that in the future a National government may acquire Harpoons before purchasing a third frigate.
Australia is New Zealand's Dutch uncle, and Australia presently will never allow New Zealand to disband entirely its frigate force. If New Zealand couldn't send a frigate when Australia called, I doubt whether Australia would maintain its ANZAC relationship. It was foolish for New Zealand to severe its ANZUS ties with the United States, it would be a disaster if New Zealand severed its ties with Australia.