Once colourful corals surrounding a Great Barrier Reef island have turned a ghastly shade of brown, with new images showing how algae has impacted the natural wonder.

WWF Australia says the photos were taken last month at Lizard Island, the "ground zero" of a mass bleaching event that's killed an estimated 22 per cent of the reef's corals.

The aftermath of the bleaching event is evident around Lizard Island on the Great Barrier Reef.

"Some people see coral bleached white and think it looks pretty. But this is what follows - it's literally an attack of the slime," WWF Australia spokesman Richard Leck said.

James Cook University PhD candidate Laura Richardson said many of the corals near Lizard Island were a ghastly sight when she surveyed the area in April and May.