Aerial photographer Colin Douch was flying over Kiama's Surf Beach on Monday when he spotted the salmon and the sharks.

"I couldn't believe it when I saw them," Mr Douch said. 

"I reckon one of the sharks would have been at least 12 foot [3.7 metres]. 

"So I rang my neighbour and he went to Kiama Surf Club and alerted the life guards and they closed the beach and were appreciative of the warning as they couldn't see the shoal from the beach."

As the beach was being closed, one surfer took to the waves anyway.

"He was surprised that while he was at the beach one surfer totally ignored the warnings and went into the water," said Mr Douch. 

Marty Garwood, a marine biologist at the Sea Life Sydney Aquarium who feeds sharks almost every day, said he wouldn't advise getting anywhere near the "bait ball" of salmon.

"It is just common sense, don't get caught up in it. You could be denying them a meal and that could certainly have a flow-on effect for the person involved." 

Mr Garwood said that sharks were taught the technique of corralling fish into bait balls from birth. 

"They are like puppies, they progress as they age." 

The sharks begin by chasing schools of juvenile fish around reefs, working together to force them into the tight ball. "The process can take some time, then when they go for it they all lunge together." 

As they get older their targets get bigger, Mr Garwood said. 

The ball is also a protective mechanism for the school of salmon, who minimise their individual chances of getting eaten by bunching together.

For the sharks it means that each lunge has the potential to bag a larger swag of fish. 

"I work with them everyday and I certainly wouldn't be getting too close," he said. 

kiamaindependent.com.au