Significant bleaching discovered on three reefs in the far north of the world's largest coral reef ecosystem was discovered by the federal government's lead reef protection agency on Wednesday.

"That is the first time we've seen significant bleaching so far this summer," said David Wachenfeld, chief scientist with the Great Barrier Reef Marine Park Authority.

"It is a confirmation of our growing concern about what is happening out on the reef at the moment."

The consequences of climate change have already played out on the reef, the authority says, with two years of bleaching in 2016 and 2017.

There is a medium risk of bleaching in the Keppel Islands, with zero or low risk in the northern half of the marine park.

Heat stress that has built up on the far northern, central and southern parts of the reef over the summer has intensified over the past week.

"These levels of heat stress are definitely capable of causing coral bleaching and we are now at a heightened level of alertness for what is happening out there in the park," Dr Wachenfeld said.

A bleaching warning has been issued for large parts of the Torres Strait and far northern management areas of the marine park, where significant bleaching across multiple hotspots is likely.

Most of the area covered by the marine park was 0.5 to 1.5C above average as of February 11, with some central and southern parts being 2 to 3C warmer.

"February is the hottest month of the year on the reef so these anomalies are really very concerning," Dr Wachenfeld said.

The reef authority has been told of bleaching in other areas and is sending staff to survey the damage.

Further heat stress is expected over the next few weeks as temperatures remain high.

Australian Associated Press