These are the highest status protected land areas administered in New Zealand, under the National Parks Act 1980. Despite strenuous efforts to tweak the legislation to allow the boundaries to include at least the intertidal beaches (e.g. on the edge of Abel Tasman National Park), and at best the subtidal areas largely surrounded by national park (e.g. within the fiords of Fiordland National Park), no change has been possible legally or politically.
It is not possible to have a Marine National Park in New Zealand without a separate empowering Act, and even then it would hardly be worth the effort because it would not provide any more protection than a Marine Reserve. This is why some of us refer to marine reserves as “national parks in the sea”, even though that is not legally possible here (unlike in Australia).