Corals in the northern sections of the Great Barrier Reef are decomposing at an alarming rate due to mass bleaching. Powerful images released by the Ocean Agency show once-pristine and colourful corals in Lizard Island rotting away, covered by brown slime and seaweed.

"It was a deeply disturbing sight," says Richard Vevers, chief executive of the not-for-profit Ocean Agency. "The white hard corals had turned brown – they were dead and covered in algae. They looked like they'd been dead for years. The soft corals were even more shocking. They were in a state of decomposition and were literally dripping off the rocks.

"It was followed by a deeply disturbing smell as soon as we got out of the water. We smelt of the rotting flesh of animals. It was an experience we will never forget."

The devastating photos bring to focus the need to urgently tackle climate change, Vevers adds.