Corals in the Great Barrier Reef have a lower tolerance to heat stress than expected, contributing to a permanent transformation of the mix of species in some of most pristine regions, a team of international researchers has found.

The scientists examined the impact of the 2016 marine heatwave that alone caused the death of about one-third of the Great Barrier Reef corals, mostly centred on the northern third section.

They studied how much abnormal heat triggers bleaching, the additional heat that killed the corals, and the accumulation needed to cause "an ecological collapse in the transformation of species", said Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, and the lead of author of the paper published Thursday in Nature.

The thresholds "are lower than we thought they would be", Professor Hughes told Fairfax Media.