Scientists have warned swathes of the Great Barrier Reef have suffered coral bleaching, with new images showing the extent of the damage.
The Climate Council says underwater footage from Heron Island to Hervey Bay in north Queensland shows stark white coral bleaching stretching across 1100km after the latest marine heatwave.
CoralWatch Project Manager Diana Kleine has been visiting Heron Island for 25 years and says it is the worst bleaching event she has seen.
"Heron Island has luckily escaped several bleaching events in the last couple of years but the way it is looking now is just devastating," she told AAP.
She says some sites have up to 80 per cent bleaching with the water temperature reaching up to 30C.
One coral she saw which was four metres wide and has taken thousands of years to grow was completely bleached.
Bleaching is caused by heat stress and it can kill corals.
While bleached corals often recover, heat stress leaves them more susceptible to diseases that can kill them.
Reproduction is also affected.
Another marine heatwave has been forecast by reef monitoring organisation National Oceanic and Atmospheric Administration (NOAA).
It is set to impact an area spanning the northern Great Barrier Reef to southern Coral Sea Islands in the coming weeks.
The NOAA recorded Alert Level 2 conditions in February, which indicate the risk of reef-wide bleaching and the mortality of heat-sensitive corals.
Ms Kleine says marine temperatures will need to cool in the coming weeks otherwise it will only be a matter of time before the bleached corals die.
The Climate Council - a not-for-profit climate information organisation - says the coral's current status could lead to another mass bleaching event declaration.
"As ocean temperatures continue to increase, our precious Great Barrier Reef is in grave danger," Climate Council's Professor Lesley Hughes said.
"Our focus must be on limiting further harm as much as possible."
It may take a decade for the reef to recover from a severe bleaching event, he warned.
Climate Council reef expert Dr Dean Miller says as the prevalence of marine heatwaves rises, more vulnerable coral species will become stressed and bleached.
"It's not just about how many corals are bleaching, but that the ones most at risk are suffering," he said.
"This stress is affecting corals of all sizes, from the largest ones that have survived past bleaching events to the smallest, youngest corals."
The Climate Council has called for environmental protection laws to be overhauled and to cease gas and coal projects which they say endangers the reef.
"Unless this law is fixed to make climate pollution a core consideration, the Great Barrier Reef will continue to deteriorate before our children's eyes," Climate Council CEO Amanda McKenzie said.
