Some 54 checks by the reef's Marine Park Authority off Mission Beach, about midway between Cairns and Townsville, found 60 per cent of sensitive coral species were already bleaching after 12 months of sustained abnormally warm temperatures.

"There's enough bleaching there to tell us that it is a significant heat-stress event," Russell Reichelt, the authority's chairman, said on Saturday. "There's the risk there of widespread bleaching leading to further mortality."

The World Heritage-listed reef last year suffered its worst bleaching event on record, with northern regions losing as much as 80 per cent of corals.

Many of the big tourist sites were spared the worst of the bleaching or recovered quickly, but this year the heat stress is closer to Cairns and other popular sites, as Fairfax Media reported earlier this month.

"It's the first time we've been getting a big bleaching event two years in a row," said Richard Fitzpatrick, an Emmy Award-winning underwater cameraman, who recently returned from Vlasoff Reef, north-east of Cairns.

The bleaching is evident at places where Mr Fitzpatrick filmed sequences for the Great Barrier Reef series led by David Attenborough, the UK naturalist.

"We've started to see the first mortality," Mr Fitzpatrick said.

If waters stay too warm for too long, corals expel the zooxanthellae algae living in their tissues that provide as much as 90 per cent of the energy they need to grow and reproduce. . The corals then bleach and face increased risks of disease, and those that survive can take years to recover.

'Alarming'

Dr Reichelt said the authority would survey other parts of the reef to see how far bleaching has spread.

The ARC Centre of Excellence for Coral Reef Studies is also preparing to repeat aerial surveys of last year to monitor changes, Terry Hughes, the Townsville-based centre's director, said.

"The 2017 bleaching is still building as we approach the summer peak temperature," Professor Hughes said. "Hopefully, it won't be nearly as bad as last year."

"It's alarming that the reef is bleaching so soon again, giving no time for recovery from the huge losses of corals in the northern third of the Reef in 2016," he said. "The scary part is that 2017 is not an El Nino year – and the period between these bleaching events is getting shorter, too short for recovery."

Imogen Zethoven, campaign director of the Australian Marine Conservation Society, said "we are running out of time" to save the reef, particularly from climate change.

Ms Zethoven singled out on-going support for the Adani-owned Carmichael coal mine that, over a projected 60-year life, would result in 4.7 billion tonnes of carbon dioxide, adding to global warming.

"There's an imminent risk of another severe bleaching event," she said. "There is no hint from the federal government that they are responding to this as a national emergency."

'Natural wonder'

Josh Frydenberg, the federal energy and environment minister, dismissed the threat to the reef from the Carmichael mine, highlighting its location rather than the emissions burning its output will produce.

"If you're talking about Adani, that's 300 kilometres inland," Mr Frydenberg told ABC Radio on Friday. "We are concerned about increased heat stress on the reef, but we are making real progress at a state and a federal level to combine our efforts to improve the health of the reef which is a beautiful natural wonder of the world."

He cited Australia signing up to the Paris climate agreement - with the country pledging to slice 2005-levels of pollution 26-28 per cent by 2030 - the 2020 Renewable Energy Target, and the Reef 2050 Plan.

However, Mark Butler, Labor's climate spokesman, said the government's own data showed Australia's carbon emissions "rising as far as projections go to 2030".

"Nothing short of real strong action, both around the reef and nationally to tackle climate change, will do," he said, noting as many as 70,000 jobs relied on tourism in the region. "That is not what we've seen after over three years of Liberal government."