The world's third-largest fully protected marine area was announced on Monday during the United Nations General Assembly in New York.

Kermadec Ocean Sanctuary northeast of New Zealand, at 239,383 square miles, will be twice the size of New Zealand's landmass and 35 times larger than all of the country's existing marine reserves combined.

Prime Minister John Key announced the creation of the sanctuary, saying it will preserve important habitats for seabirds, whales, dolphins, endangered sea turtles and thousands of other species.

"As well as being home to a wide range of marine species, the Kermadec region is one of the most geographically and geologically diverse areas in the world," Key said.