MORE than 90 per cent of Great Barrier Reef coral surveyed this year was bleached in the fourth such mass event in seven years in the world’s largest coral reef ecosystem, Australian government scientists said.

Bleaching is caused by global warming, but this is the reef's first bleaching event during a La Nina weather pattern, which is associated with cooler Pacific Ocean temperatures, the Great Barrier Reef Marine Park Authority said in its annual report released this week that it found 91 per cent of the areas surveyed were affected.

Bleaching in 2016, 2017 and 2020 damaged two-thirds of the coral in the famed reef off Australia's eastern coast.

Coral bleaches as a heat stress response and scientists hope most of the coral will recover from the current event, said David Wachenfeld, chief scientist at the authority, which manages the reef ecosystem.

“The early indications are that the mortality won’t be very high,” Mr Wachenfeld said on Wednesday.

“We are hoping that we will see most of the coral that is bleached recover and we will end up with an event rather more like 2020 when, yes, there was mass bleaching, but there was low mortality,” Mr Wachenfeld added.

The bleaching events in 2016 and 2017 led to “quite high levels of coral mortality,” Mr Wachenfeld said.

Simon Bradshaw, a researcher at the Climate Council, an Australia-based group that tracks climate change, said the report demonstrated the reef's survival depended on steep global emission cuts within the decade.

“This is heartbreaking. This is deeply troubling,” Mr Bradshaw said. “It shows that our Barrier Reef really is in very serious trouble indeed.”

Last December, the first month of the Southern Hemisphere summer, was the hottest the reef had experienced since 1900. A “marine heatwave” had set in by late February, the report said.

A United Nations delegation visited the reef in March to assess whether the reef’s World Heritage listing should be downgraded due to the ravages of climate change.

The environmental group Greenpeace Australia Pacific said today that the extent of the latest bleaching was “another damning indictment of the [Scott] Morrison government which has failed to protect the reef and exacerbated the problem through its support of fossil fuels.”