Authorities monitoring the Great Barrier Reef Marine Park have increased the coral bleaching threat level after divers found widespread loss of coral.

Great Barrier Reef Marine Park Authority (GBRMPA) chairman Dr Russell Reichelt said the area around Lizard Island, north of Cairns, and sites further north, had been hit hardest.

It warned there was a high risk of mass coral bleaching on the reef this month due to the hot, dry conditions associated with the El Nino weather system and high sea surface temperatures.

"This is the result of sea surface temperatures climbing as high as 33 degrees Celsius during February," Dr Reichelt said.

"In the far north, the surveys found severe bleaching on inshore reefs, along with moderate bleaching on mid-shelf reefs."

Bleaching dramatically increased in past two weeks

Since earlier this month, divers from the GBRMPA have been working with the Queensland Parks and Wildlife Service and other scientists to survey the extent and severity of bleaching.

Dr Reichelt said GBRMPA would ramp up monitoring of the mass bleaching event in the coming days.

Lyle Vail, who runs the Australian Museum's Lizard Island Research Station north of Cairns, said bleaching had increased dramatically in the past two weeks, especially among shallow water corals.

"A couple of weeks ago you'd look around in the Lizard Island lagoon and see at least 50 per cent of corals were stressed to some level, but none had died," he said.

"Now you look around and see all the corals are highly stressed and a couple of colonies have died."

Earlier this month, Mr Vail said the bleaching was the worst to hit the island in more than 15 years.

Bleaching reported up to 30kms away from Lizard Island

He said it would take time for the coral to recover when cooler air and sea temperatures eventually arrived.

"Corals aren't going to miraculously recover. It takes them time, if they're going to survive, to get over such a stressful event," he said.

"It will take many weeks for the coral to get as close as it can to previous condition.

"The problem with having these high levels of stress is it will affect their growth and reproductive output in the future."

Mr Vail said other researchers monitoring corals in the area had reported bleaching up to 30 kilometres away from the island.

He said there were signs the worst weather was over.

"We're starting to see the sea temperatures go down gradually after a week of cloud cover and cooler air temperatures," he said.

'Not out of the woods yet'

Director of the ARC Centre of Excellence for Coral Reef Studies Professor Terry Hughes said the mass bleaching was a "real tragedy".

"After months of El Nino conditions, we had hoped that cloudy weather in the past few weeks would quench the overheating of the Great Barrier Reef along its entire length," said Professor Hughes.

He said extensive aerial surveys would begin this week, similar to those done during mass bleaching events in 1998 and 2002.

Queensland's Environment Minister Steven Miles said staff on the state and federally funded Reef Ranger were assisting with monitoring efforts.

"We're not out of the woods and I hope we will not see any broad-scale bleaching events like those that endangered the Reef in 1998 and 2002," he said.

"Events like this serve as a wake-up call for everyone that climate change is real and affecting us now."