As much as half the Great Barrier Reef may have died in the back-to-back bleachings over the past two years.
But the head of the authority in charge of the reef says the actual extent of die-off is tricky to calculate because some parts are growing well.
The Great Barrier Reef Marine Park Authority believes about 30 per cent of coral, in the reef's northern part, died last year in bleaching caused by warmer ocean waters, chairman Russell Reichelt told a Senate committee on Monday.
Surveys after this year's bleaching are still being done but initial observations suggest 20 per cent of coral - mainly in the central area - is dead.
"Don't think of these figures as the net amount of coral on the Barrier Reef because there are quite big movements upwards as well as downward," Dr Reichelt told the senators at an estimates hearing in Canberra.
The southern part of the reef had grown by about 40 per cent in recent years because it hadn't been hit by cyclones or bleaching - but it was likely it would suffer from those in the future.
Dr Reichelt said the bigger picture question was the coral's resilience in the face of bleachings, tropical storms and other threats such as the crown of thorns invasions.
"It depends on the frequency of these major impacts and the concern is the frequency could well be increasing and the recovery time will be insufficient," he said.
"If the recovery time is very short, there won't be a lot of coral."
The best science suggests global warming needs to be limited to 1.5 degrees to allow a good survival rate for coral.
There had already been a 0.7 degree warming over the past century, Dr Reichelt said.
"I draw the public's attention and the committee to the fact the unprecedented back-to-back bleaching we've seen is occurring on a fraction of a degree (rise in temperature)," he said.
"The safe levels (of warming) for coral reefs, probably we've passed already."
The authority is looking at ways to boost the resilience of corals and encourage more rapid regrowth.
© AAP 2018