But the reef was actually facing a twin threat, with chemical run-off from farms along the coast of Queensland state threatening to trigger an attack by predatory Crown-of-Thorns starfish, who thrive on farm waste, the environment watchdog WWF said in a report.

"It is reducing the reef's resilience to climate change. The risk is farm pollution will feed another outbreak of this invasive species, which devastates reefs and can halve coral cover," WWF water expert Nick Heath told Reuters.

The starfish, which lives on tiny living polyps which make up the reef, can each wipe out up to six metres of coral each year and scientists believe agricultural run-off can help it to thrive.

WWF said as many as 700 of the Great Barrier Reef's 3000 coral outcrops were in danger because of human activity in water catchments along the coast and pesticides used by the sugar cane industry.

"Areas damaged by coral bleaching recover more slowly and less fully where there is a water quality problem," Heath said.

The reef was declared a marine park in 1975 and a management plan introduced in 2004 boosted its highly protected areas to a third of its 340,000 square kilometres (133,000 square miles).

But WWF said the Australian government needed to spend at least A$300 ($241 million) on urgent on-farm measures to cut pollution runoff.

"There are some marvellous examples of good farm practice, but they are not the majority," Heath said.

The reef is home to more than a third of the world's soft corals, more than 1,500 species of fish and six of the world's seven marine turtle species.