I think part of the problem is that NZ hasn't really got a big shipbuilding industry. If it built its own ships and communities were reliant on that industry to provide work then you might see a lot more public support for spending money on new ships. As it is most of that money is disappearing overseas.Yes, one thing that has surprised me about Australia, despite the many changes of leadership over the last ten years, no major cuts to defence! Something I envy about them,apart from the weather, the better wages, etc.
Perhaps if there were some agreement with Australia in which NZ could build blocks for both NZ and Australian ships. Once NZ is locked into that supply chain then the general public ... or at least the voters reliant on the shipbuilding industry ... will be less resistant to defence spending.