The coral bleaching threat level on the Great Barrier Reef Marine Park is expected to increase next week, with marine experts saying it is already at its worst.

University of Queensland marine researcher Professor Justin Marshall, who has been visiting the reef for 30 years, said the bleaching in the northern section of the World Heritage Area was the worst he had ever seen.

Loading

"I'm yet to see a healthy coral [near Lizard Island], there's no coral that hasn't been bleached," he said.

"I'm very worried I'm witnessing the death of a very large part of this reef system. I'm extremely worried and upset.

"I used to bring my kids here, I'd be ashamed to bring them here now to be honest."

On Monday, the Great Barrier Reef Marine Park Authority (GBRMPA) increased the coral bleaching threat level after discovering "severe bleaching on inshore reefs … and moderate bleaching on mid-shelf reefs".

Professor Marshall joined other researchers who had been out on the reef in warning the threat level could increase again.

"I think we should expect to go to stage three [threat level] next week. I would say the chances are high," he said.

Relief in sight as bleaching event almost over

Professor Terry Hughes from the ARC Centre of Excellence for Coral Reef Studies said there was growing evidence the worst of the bleaching had hit north of the World Heritage Area.

"We've just had a boat come back from a week off Townsville, where researchers saw mild to moderate bleaching, and there wasn't much to see at Heron Island," Professor Hughes said.

"There's a strong north-south gradient in the bleaching."

He said it was likely the worst was nearly over.

"The rain will cool the temperature down, so we should be very close to peak bleaching now," he said.