La Nina is likely to bring more tropical cyclones, rainfall and flooding than average over the next four months, the Great Barrier Reef Marine Park Authority says.

The world's largest coral reef system could be slammed by big waves, warmer ocean temperatures and pollutants washed into the marine environment by floodwaters.

"The summer months are a period of elevated risk for the Great Barrier Reef," chief scientist David Wachenfeld told reporters on Thursday.

"These are the months when we will see extreme weather events."

Dr Wachenfeld said the reef was likely to be hit with warmer than average air and surface water temperatures and higher rainfall during December and January.

"Beyond that into February and March, which are still part of the season of peak risk, ... there is still great uncertainty about what we expect to see," he said.

"On the one hand we have the forecast of warmer than average conditions, but at the same time because it is a La Nina phase there is the general forecast of a larger than average number of cyclones and wetter than average conditions."

Climate change is making it harder to forecast weather on the reef, which is now 0.8C warmer on average than 100 years ago, Dr Wachenfeld said.

"The entire system is changing. Overall, we are getting more concerned about extreme weather events ... those kinds of events are only going to get worse," he said.

Climate change remains the biggest threat to the reef and robust global action to reduce greenhouse gas emissions was needed, Dr Wachenfeld said.

"The world is warming slowly but it is not so much the background warming that causes problems for the reef, it's the extreme weather events that get driven by that - marine heatwaves, more severe cyclones, extreme floods," he said.

"Corals are highly sensitive to temperature changes over summer and our climate continues to change each year."

Coastal development, water quality and fishing also continue to test the reef's health.

"We need to act on climate change but we need to act locally as well."

Despite recent climate-driven impacts on the reef - such as the extreme coral bleaching in 2016, 2017 and 2020 - it remains a beautiful, vibrant and diverse ecosystem, Dr Wachenfeld said.

La Nina happens when equatorial trade winds become stronger, changing the ocean's surface currents and drawing cooler water up from the depths.

The enhanced trade winds also cause warmer surface waters to the north of Australia, potentially leading to heavier rainfall and more tropical cyclones than average.

Australian Associated Press