Millions of corals on the Great Barrier Reef were 'cooked' during a scorching summer in the northern region, according to scientists.

The underwater heatwave eliminated a huge number of different species of coral during a process which expelled algae after the polyps were stressed.

'When corals bleach from a heatwave, they can either survive and regain their colour slowly as the temperature drops, or they can die.

There are variations in the appearance of severely bleached corals. Pictured: the coral displays pink fluorescing tissue signalling heat stress

A researcher accesses minor damage at Day Reef (pictured) on the Great Barrier Reef following the March 2016 mass coral bleaching event

'Averaged across the whole Great Barrier Reef, we lost 30 per cent of the corals in the nine-month period between March and November 2016,' said Professor Terry Hughes from James Cook University said.

Prof Hughes who acts as the Director of the ARC Centre of Excellence for Coral Reef Studies at JCU said his team was very surprised to see a quarter of the corals die in just two to three weeks during the March heatwave.

Scientists researched the entire reef by analysing water surveys at various locations along its 2,300-kilometre distance, and combined insight with aerial data and satellite monitoring.

A severely bleached branching coral amongst the minimally bleached boulder coral (pictured)

A bleached Acropora colony (pictured) was discovered by researchers during the study

Results showed 29 per cent of the 3,863 reefs which make up the world's largest reef system lost 'two-thirds or more of their corals', which dramatically impacts the ability of the reefs to maintain full ecological abilities.

'The Great Barrier Reef is certainly threatened by climate change, but it is not doomed if we deal very quickly with greenhouse gas emissions.

'Our study shows that coral reefs are already shifting radically in response to unprecedented heatwaves,' said Prof Hughes.

The team warn that if changes are not made to consider climate change it will have a huge effect on tropical reef ecosystems and, therefore, a detrimental impact on the benefits those environments provide to populations of poor nations.