New Zealand has unveiled plans to create a South Pacific marine sanctuary the size of France, saying it would protect one of the world's most pristine ocean environments.

Prime Minister John Key said the Kermadec Ocean Sanctuary would cover an area of 620,000 square kilometres (240,000 square miles) about 1,000 kilometres (620 miles) off New Zealand's northeast coast.

Announcing the plans at the United Nations in New York on Monday, Key said the Kermadec area was home to thousands of important species, including whales, dolphins, seabirds and endangered turtles.

"(It) is one of the most geographically and geologically diverse areas in the world," he said in a statement.

"It contains the world's longest underwater volcanic arc and the second deepest ocean trench at 10 kilometres deep."

The sanctuary will prevent fishing and mineral exploitation in an area where scientists regularly discover new marine species.

Conservation groups applauded the move, saying it added to a network of protected areas in the Pacific that now covered more than 3.5 million square kilometres.

"We congratulate the government for taking decisive action to protect this incredibly special area from mining and fishing," World Wildlife Fund New Zealand chief executive Chris Howe said.

"This decision puts New Zealand back at the forefront of marine protection on the global stage."

-- Globally important --

Scientists also applauded the decision to ban all forms of resource extraction from the marine zone, which is remote and largely unsurveyed or scientifically sampled.

"The Kermadec region is an exceptional place and is very much in need of protection before large-scale exploitation really commences," said Jonathan Gardner from Victoria University's school of biological sciences in Wellington.

"As a contribution to large scale marine conservation the Kermadec Ocean Sanctuary is globally important."

The variation in the area, which spans from tropical to sub-tropical waters, means it boasts a high diversity of habitat types including active undersea volcanoes, ridges and the Kermadec Trench itself, said Malcolm Clark from New Zealand's National Institute of Water and Atmospheric Research.

Some organisms are only found in that part of the world while others, such as whales, include it in their migration routes, he said.

"We are just beginning to understand the abundance of life there," said Bronwen Golder from the Pew environmental organisation.

"But we know that creating this marine sanctuary will safeguard rare habitats and species critical for healthy ecosystems throughout the South Pacific."

New Zealand's Environment Minister Nick Smith said oceans were the new frontier for environmental protection, making up 72 percent of the globe and home to half of the world's species, but with only two percent protected despite pressures from over-fishing, mining and pollution.

Conservation efforts are underway in the Pacific to create a network of marine parks across the region to ensure one of the world's last pristine ocean ecosystems is managed sustainably.

In 2012 the Cook Islands unveiled a 1,065 million square kilometre marine park while Kiribati and Tokelau have also declared huge protected zones.

New Zealand said it hoped to have legislation enacted in parliament to ensure the sanctuary in place by October 2016.