Experts from the Great Barrier Reef Marine Park Authority say parts of the central and south areas are 1.5 to 3C warmer.

Federal Environment Minister Sussan Ley says there has been anecdotal reports of isolated bleaching events.

The authority's chief scientist David Wachenfeld has assured her there is no confirmation of mass coral bleaching.

"But (he says) that the current weather pattern, in what is the area's hottest month of the year, is a concern," she said.

"He notes that this pattern can change quickly and that it may take several weeks to establish an accurate picture."

The reef is being monitored by divers, vessels and aerial surveillance.

Bleaching is a stress response that causes coral to lose colour, with some dying while others manage to survive.

GBRMPA last year warned the outlook for the reef was "very poor" and would not improve unless global action was taken to address climate change.

A study from last year showed living coral cover on the reef has almost halved since the late 1990s.

Separate analysis has shown the unprecedented consecutive bleaching events on the reef in 2016 and 2017 changed the physiology of tougher coral.

The research suggested the changes made the coral more resilient to future stress.

Australian Associated Press