The government has formally declared the Kermadec region an ocean sanctuary.
The sanctuary is about 1000km north east of New Zealand, in the South Pacific Ocean.
Mining, prospecting and fishing are to be banned in the sanctuary which, at 620,000 square kilometres, is more than twice the size of New Zealand's land area.
Prime Minister John Key made the announcement at the UN General Assembly in New York.
The area concerned is one of the most geologically diverse marine habitats in the world, and serves as a breeding and feeding ground for a variety of marine mammals, fish and invertebrates, including dolphins, turtles and coral.
It is home to the world's longest chain of submerged volcanoes, and the second deepest ocean trench, with a depth of 10 kilometres.
Conservationists are hailing the news, which a Forest and Bird spokesperson said put New Zealand at the forefront of marine protection, on the international stage.
WWF New Zealand chief executive Chris Howe said it was a major achievement and testament to many years of hard work by thousands of New Zealanders.
Pew Global Ocean Legacy Kermadec campaign director Bronwyn Golder said it was an announcement all New Zealanders could take pride in.
Environment Minister Nick Smith said the sanctuary, which is 35 times bigger than all New Zealand's 24 marine reserves added together, "will be a large area in which nature rules."
He said that just as New Zealand had been a world leader in establishing national parks, it needed to do the same in regard to its marine areas.
"It's part of that global effort for us to lift our game in managing the oceans of the world," he said.
Mr Smith said the sanctuary would be monitored by the navy and satellite technology.
He said there could be an economic cost to the mining industry, but that it was important to protect this patch of ocean before it was subject to development pressures.
Though the government wanted the jobs and export opportunities from the country's huge ocean resource, he said it also believed really special areas must be protected.
Fishing company says decision based on poor knowledge
Fishing company Solander Seafood and Fishing said the government's decision to ban fishing the newly-created ocean sanctuary put a stop to any idea the country's tuna fishery could grow.
The company said if the tuna industry was to expand, the government should allow surface fishing in the area.
Managing director Charles Hufflett said the zone was already protected by an industry agreement not to fish on the seabed.
He said the sanctuary decision would restrict the local tuna industry to a small coastal fishery and was based on poor knowledge of the various fish species in the area.
"They know an awful lot about moo cows and baa lambs, but nothing really about the different fish species," he said.
Long campaign for Greens
The Green Party said it had been a long campaign to get the government to formally declare the region a sanctuary.
Environment spokesperson Eugenie Sage said it was a significant decision, as conservation groups had been trying to have the area protected for years.
Ms Sage said establishing the sanctuary sent a strong signal that New Zealand was a leader in marine conservation.