The Labour Party says that if it were elected it would extend the marine reserve around the Kermadec Islands by more than 200 nautical miles.
The party's conservation spokesperson, Ruth Dyson, says the expansion would make the total size of the reserve 620,000 square kilometres.
She says that would mean the reserve would become one of the first of the Exclusive Economic Zone to support an entire ecosystem.
Ms Dyson says the Kermadec Islands are a remarkable part of the planet, with underwater volcanoes and incredible marine diversity.
The party will release its full conservation policy later in the week.