Sixth mass bleaching event strikes reef since 2016

Extensive coral bleaching has been confirmed at Australia's Great Barrier Reef for the sixth time in less than a decade as warming oceans and severe floods wreak havoc.

The northern regions of the world's largest coral reef system have suffered bleaching due to prolonged exposure to elevated temperatures, Australian government agencies reported on Wednesday.

It marks a second consecutive year of mass bleaching at the World Heritage-listed site that's vulnerable to climate change like all coral reefs around the world.

Corals turn pale in response to heat and other stressors.

They can recover, though mortality is more likely the longer water temperatures stay high.

The Great Barrier Reef Marine Park Authority, Australian Institute of Marine Science and CSIRO have reported the mass bleaching event, but further monitoring and assessment are needed before coral mortalities are confirmed.

The reef has further incurred damage from record rainfall in the northern and central regions, with the influx of freshwater changing water salinity and bringing contaminates that harm corals.

Of the 162 inshore and mid-shelf reefs surveyed, 41 per cent recorded medium to high bleaching.

The worst heat stress was recorded from Cooktown to Cape York.

World Wide Fund for Nature-Australia head of oceans Richard Leck said the reef was "crying out for climate action".

"Bleaching is becoming the new normal," he said.

"The next term of government could be the last opportunity to give our reef a fighting chance."

Australian Marine Conservation Society Great Barrier Reef campaigner Simon Miller said the government response should not end at monitoring.

"The number one thing the Australian and Queensland governments must do is adopt reef-safe climate policies," Mr Miller said.

"That means cutting climate pollution by 90 per cent by 2035 and stopping approvals for new fossil fuel projects."

Western Australia's coral ecosystems have also been enduring a mass bleaching event from unprecedented heat stress, with reefs around the world under pressure from rising ocean temperatures.

Roughly a quarter of all marine life rely on coral reefs at some stage in their life cycle and many coastal communities rely on them for tourism dollars and food security.