Between 70 and 90 per cent of coral in the shallow water around Christmas Island are bleached due to rising ocean temperatures, researchers say.

Over the past month, sea temperature around the island has jumped from the usually 28 degrees to above 31 degrees.

It has pushed corals to expel the algae that give these organisms their vivid colour, and today much of the reef has been bleached stark white.

The event is part of the same rise in ocean temperatures that has caused widespread coral bleaching on the Great Barrier Reef.

Before: This photo shows the water ringing Christmas Island before coral bleaching. ( Curtin Aquatic Laboratory )

Jean Paul Hobbs and a team of divers from the Curtin Aquatic Laboratory have been assessing the scale of bleaching.

"This is about as bad as it gets bleaching-wise," he said.

Christmas Island is formed around on a former volcano, and reefs extend all around its fringe.

Dr Hobbs has been surveying the island's reefs as part of a national effort to determine the impact of this bleaching event.

"The coral extends down 60 to 70 metres and there is bleaching all the way down to those depths," he said.

"It indicates what we are seeing is a widespread event, rather than the patchy event they had in the past."

After: The white patches indicate coral bleaching in the shallow waters off Christmas Island. ( Curtin Aquatic Laboratory )

Tourism threatened

Tourism is one of the major employers on the island, and the coral bleaching could threaten the livelihood of the island's 1,500 residents.

Linda Cash from the Christmas Island Tourism Association is carefully monitoring the event.

"Without these natural resources I don't think those in policy understand the impact that it has on a place like Christmas Island, who are pretty much going to be relying on tourism for its future," she said.

In 1998, Christmas Island suffered from another major coral bleaching event. On that occasion much of the reef recovered.

Ms Cash said residents were hoping for a similar outcome this year.

Fish in the waters off Christmas Island. ( Curtin Aquatic Laboratory )

"We are certainly very hopeful at this stage that it is just a bleaching event, and that the algae come back into the reef," she said.

"We did have fantastic recovery back then; all we can hope for is the same result this time."

Coral bleaching does not necessarily result in coral death; in many cases the algae that reside in corals are able to recover.

Dr Hobbs has started laying marks across the reef to measure what percentage of coral dies.

"Fortunately the water has cooled down in the past week and it's gone from about 31 to 29 degrees.

"This will give the corals a chance to recover.

"This is why we are interested in monitoring the corals are going to recover with this cool water that has arrived, and how many will die because the previous warm water was just too much for too long."