The Pacific island nation of Kiribati has created the world's largest protected marine reserve, conservation groups say.

They describe it as a California-sized wilderness, brimming with reefs, fish and birds.

The Phoenix Islands Protected Area, covering 410,500 square kilometres, is one of the planet's last intact coral archipelagos and is threatened by over-fishing and climate change, the groups say.

It lies near the equator about half way between Fiji and Hawaii.

"The creation of this amazing marine protected area by a small island nation represents a commitment of historic proportions," said Russell Mittermeier, president of Conservation International.

The US-based group, along with the New England Aquarium, is helping the Kiribati government develop a management and funding plan for the largely uninhabited area.

Studies led by the US aquarium have found more than 120 species of coral and 520 species of fish, some new to science.

The area also has some of the most important sea bird nesting sites in the Pacific, large fish populations and sea turtles, the aquarium and Conservation International say.

The protected zone is more than double the area Kiribati originally pledged to protect at a UN biodiversity conference in Brazil in 2006.

"The new boundary includes extensive seamount and deep-sea habitat, tuna spawning grounds and as yet unsurveyed submerged reef systems," said Greg Stone, the aquarium's vice-president of global marine programs.

Kiribati says it needs more money to pay for surveillance against illegal fishing as well as develop a trust fund, possibly as large as $US100 million ($A111.6 million), to pay for running costs and compensate the government for lost income from commercial fishing licences.

"A major part of the operational cost is the surveillance and we have a patrol boat donated by Australia," Tebwe Ietaake, secretary of Kiribati's environment ministry, told Reuters.

"We are also looking at the cooperation of Australia and New Zealand in aerial surveillance flights over the region," he said.

He said the government would still allow subsistence fishing by local fishing communities.

Conservation International said it was crucial to protect the area from overfishing because healthy reefs and fishing grounds helped the area better withstand the impacts of climate change.

The reefs were already facing the threat of warming seas, which has caused repeated coral bleaching around Kiribati and elsewhere in the Pacific, such as the Great Barrier Reef.

Parts of Kiribati, too, are already suffering from the effects of rising seas, including coastal erosion in the vast archipelago and salt water intrusion into fresh water supplies.

The expanded Phoenix islands reserve is closely matched in size to the Papahanaumokuakea Marine National Monument in Hawaii, the single largest conservation area under the US flag, covering 357,000 square kilometres of the Pacific Ocean.

Australia's Great Barrier Reef Marine Park covers about 345,000 square kilometres, and extends more than 2,300km along the Queensland coast.