30/05/2016 11:45 AM AEST | Updated 15/07/2016 12:53 PM AEST

Coral Bleaching 'Death Toll' Is Worst North Of Cairns

First comes the heat, then coral bleaching, then death.

Coral bleaching has killed one in three corals on the Great Barrier Reef's central and northern regions while stretches south of Cairns are faring better.

The study, by the same scientists who shocked the world with reports of 93 percent coral bleaching on the Great Barrier Reef, has gone back to those reefs to see how many corals had actually died.

How does coral bleaching kill coral?

Coral hosts tiny algae in its tissues called zooxanthellae that produce its food as well as its colour.

When waters become too hot or cold, the coral becomes stressed and its metabolism and reproductive system break down.

At this point, it cannot process the oxygen created by the zooxanthellae, so it expels them.

If the temperature returns to normal, these tiny creatures can re-enter the tissues, or if most but not all algae have been expelled, the remaining few can keep the coral alive.

If the temperatures remain extreme and the sun continues shining, the coral dies.

James Cook University ARC Centre of Excellence for Coral Reef Studies director Terry Hughes said 35 percent of corals were now dead or dying on 84 reefs between Townsville and Papua New Guinea.

"Some reefs are in much better shape, especially from Cairns southwards, where the average mortality is estimated at only five percent," Hughes said in a statement.

Researcher Mia Hoogenboom said these southern reefs could start to look better sooner than initially thought.

"We expect these more mildly bleached corals to regain their normal colour over the next few months," Hoogenboom said.

Ho New / Reuters
Marine activist Suzanne Kavanagh swims above coral bleaching on the Great Barrier Reef.

The coral bleaching event began in summer 2015 when ocean temperatures increased as the El Nino weather system hit -- bringing a column of heated, tropical water down the coast of Australia.

That, combined with record heat and dryness, created the worst coral bleaching event in history.

For Hughes, last season's coral bleaching event was not an anomaly but a sign of things to come.

"This year is the third time in 18 years that the Great Barrier Reef has experienced mass bleaching due to global warming, and the current event is much more extreme than we've measured before," Hughes said.

"These three events have all occurred while global temperatures have risen by just 1 degree C above the pre-industrial period.

"We're rapidly running out of time to reduce greenhouse gas emissions."

Meanwhile Labor has today announced a $500 million promise to protect the Great Barrier Reef.