New Zealand 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 on Monday said the Kermadec Ocean Sanctuary would cover an area of 620,000 square kilometres about 1,000 kilometres off New Zealand's northeast coast.

Announcing the plans at the United Nations in New York, 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 marine scientists regularly discover new marine species.

Environmental 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," WWF New Zealand chief executive Chris Howe said.

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