Australian scientists are joining a global study that is trying to get a better idea of shark populations in reefs.

The three-year study will gather underwater video from 400 sites including the Great Barrier Reef off Queensland and the Ningaloo Reef off the West Australian coast.

Dr Mark Meekan from the Australian Institute of Marine Science said the data would give scientists a better understanding of how threatened sharks are.

"We know that sharks are in trouble, we just don't know how much trouble they are in," he said.

"Booming markets for shark fin around the world means sharks are declining in abundance."

Dr Meekan said the survey would also give authorities ideas on how to protect the animal, to see what is going wrong or right.

"What we'd like to know is what are the places where sharks are intact, how do we preserve those places and what are we doing right in terms of management in terms of people that are actually preserving populations," he said.

Dr Meekan said the study would also help researchers understand how reefs work without the predators.

Aquarist Laura Colton, who works with reef sharks at public education centre Reef HQ, said the data could help boost efforts to save shark species threatened with extinction.

"When you have the data to back it up, it's much easier to show what you're talking about, [it] does have that importance to it," she said.

The research could also change public perceptions around sharks and give people a greater understanding of the importance of sharks to the ecosystem.

"Of course there are always some people who think sharks are scary things so it's always interesting when you get to talk to them about the importance of sharks and how they fit it, and see that perception change," Ms Colton said.

The data will be collated at the end of the three-year survey and scientists will go through thousands of hours of footage.