The world's southernmost coral reef is on a "knife-edge" after warmer seas blamed on climate change bleached large parts of it for the first time, an Australian scientist warned on Wednesday.

Peter Harrison, who has been monitoring the world heritage-listed Lord Howe Island since 1993, said a two degrees Celsius (four Fahrenheit) rise in sea temperatures had drained much of the reef of its distinctive colours.

"We're hoping the vast majority of these corals will be able to recover, but at the moment this whole system is on a knife-edge and we don't know what's going to happen," Harrison told AFP.

Harrison, whose Southern Cross University team surveyed the reef about 600 kilometres (370 miles) east of Sydney this month, said the unusually warm water had travelled down from tropical seas around Australia's Great Barrier Reef.

He blamed the warmer seas on climate change and said the reef could face "an even more severe event" in the future.

"It's exactly what you predict from warming seas," Harrison said. "This is a warning of likely future increases of stress on this world's southernmost reef."

Harrison said the reef will survive if the seas return to their normal temperatures quickly enough, but could take decades to recover from a severe bleaching.

He added that the damage was also affecting other marine life, including a type of anemone which provides a home for a rare type of fish.

Bleaching occurs when overheated corals expel crucial algae that give them their colour.