New marine species may have been discovered off the north-western Hawaiian Islands after a major deep-sea exploration, scientists say.

To make the find, scientific divers descended to previously unexplored depths in the area as part of a 25-day National Oceanic and Atmospheric Administration (NOAA) research expedition.

They have returned with specimens and photographs of new records of marine life, including sea urchins, sea cucumbers, algae and reef fish.

Chief scientist Dr Randall Kosaki said the discovery of new species is likely.

"We discovered quite a few organisms that we cannot identify. It's very likely that many of them are new species," he said.

"Now we're back on shore, that's when the real work begins."

The scientists visited French Frigate Shoals, Lisianski Island, Pearl and Hermes Atoll and Midway Atoll.

Dr Kosaki said the area and its marine life are not very well understood.

"This area occupies two thirds of the Hawaiian archipelago but it's largely uninhabited and very, very remote," he said.

"So it's not a place that very many people, or very many expeditions get to visit.

An unidentified species of sea urchin may be completely new to science. ( NOAA/Robert Whitton/Bishop Museum )

"For this reason it's not very well explored even in shallow water and the primary mission of our expedition was to explore some of the deeper waters."

The mission was made possible by the use advanced diving technology study coral reefs as deep as 91 metres.

Dr Kosaki said the exploration is the deepest study done in the area he calls the "twilight zone".

"We're finding that the coral reef habitat goes deeper than most scuba divers," he said.

"Most scuba diving research is done at depths of maybe five to 30 metres. We were diving at depths of 50 to 80, or 90 metres.

The mission was made possible by the use of advanced diving technology allowing the study of the deeper coral reefs.

The rare deep-water Hawaiian Pigfish (Bodianus bathycapros) has never been seen or photographed by divers. ( NOAA/Richard Pyle/Bishop Museum )

He said scientists will now compare the specimens found with known species in museum and university collections.

During the trip, Dr Kosaki said scientists found damage caused by climate change, including reef bleaching.

He said Lisianski Island appeared to be the hardest hit with large areas of once vibrant purple coral now white and some corals dying.

"It's a habitat that's under threat from global climate change from global warming," he said.

"So we stand to lose species before we even know they exist ... so there is some sense of urgency to document much of this before it's potentially lost."