It only took a few days of unusually warm water in Sydney Harbour to bleach its coral population for the first time on record, researchers say.
Paled coral colonies were found for the first time near Manly and Middle Head during routine monitoring by PhD candidate Samantha Goyen and Dr Matthew Nitschke from the University of Technology, Sydney.
"A warm water influx happened a few weeks ago, which we think was sustained for ... (up to) a week," Dr Nitschke told AAP.
"It appears that week-long period was enough to cause bleaching."
Bleaching occurs when the coral experiences physiological stresses such as warm water or sun exposure for extended periods.
Of the two main species of coral found in Sydney Harbour, only one of them seems to be affected by bleaching, Dr Nitschke said.
Corals on upper surfaces, such as on top of boulders, that are exposed to high light levels are the most severely impacted, he said.
"If you look in crevices you find corals that are completely fine," said Dr Nitschke.
He is optimistic the corals will recover once water conditions return to normal.
"Corals can certainly recover from bleaching," he said.
"The conditions have cooled down in the harbour now and that could be enough to make sure they don't all die from this."