A 50-year-old local man has died after being attacked by a shark while swimming at a beach at Byron Bay in northern New South Wales.

Police said the man's wife was on the beach when he was bitten on the leg about 15 to 20 metres off the shore at Clarkes Beach about 10:45am (AEST) on Tuesday.

Beachgoer Mark Hickey pulled him from the water and onlookers unsuccessfully tried to resuscitate the man.

Mr Hickey said he initially thought the attack victim was a turtle.

"Then I looked closer and there was a lot of blood in the water and there was a shark circling, and I recognised that it was a person," he said.

"I ran out to about chest-height in water and grabbed the guy and dragged him into the beach."

Mr Hickey said it appeared the man had bled to death in the water.

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"There was a big gash out of his right leg, above the knee," he said.

NSW Police Inspector Bobbie Cullen said the man's wife was on the beach at the time.

"Obviously the witnesses are extremely distressed," she said.

A spokesman for the ambulance service said it was likely the man went into cardiac arrest when he was in the water.

Helicopters and jetskis were patrolling the the area and a large shark, believed to be a great white, was spotted close to the shore soon after the attack.

"It may be that we are able to push the shark out to sea if we do locate it," Inspector Cullen said

One witness, 26-year-old Daniel Toole, said he was surfing when he noticed a crowd of people yelling at him to get out of the water.

"Everyone [was] yelling at me to come in so I ran in and dropped the board, and everyone had got to him and were trying to help him," he said.

"They were giving him CPR and just trying to get him back and everyone ringing all the ambos and that, and just trying to get help. [I'm] pretty rattled."

A shark, believed to be about two metres long, was spotted in the area after the attack. ( Supplied: Westpac Life Saver Rescue Helicopter )

Local surf school operator Sean Riley said he was alerted to the attack by his coaches on the beach.

"They rang me, I was in our surf shop, and I came down here to make sure everyone was OK, and get everyone out of the water," he said.

"They basically said there was a guy down the beach, he had flippers on and was floundering around, he'd just been attacked by the shark.

"Some people on the beach got him in and the boys from the surf school got everybody out of the water and tended to the guy."

Beaches from Tallows to Belongil would be closed until at least Wednesday afternoon, lifeguards said.

"We're trying to keep everyone safe in this area," lifeguard Scott McCartney said.

"We just encourage everyone on the coast to stay out of the water in the Byron Shire area until we deem it safe for swimming."

Police said the man was pulled from the water after he was seen floating near the shore. ( AAP Dave Hunt )

He said lifeguards rarely saw sharks in the area.

But a senior lecturer in marine biology at Southern Cross University, Daniel Bucher, said there were currently many sharks in the area.

"[In] winter we tend to get more cool-water species, like big great whites will come up here at that time," he said.

"As the water warms up though, they tend to move south and start to hassle the seals down there and we get things like tiger sharks and makos and bull sharks becoming more common.

"Generally sharks attack from behind and so most shark attack victims don't see the shark coming, so there's really not much you can do."

A report was being prepared for the coroner.

Shark attacks over the past 100 years

State # Cases Fatal Injured Uninjured Last Fatality NSW 203 48 105 50 Byron Bay 2014 Qld 159 56 89 14 Palm Island 2011 WA 77 14 51 12 Gracetown 2013 SA 40 13 21 6 Goldsmith Beach 2014 VIC 32 4 18 10 Mornington Peninsula 1987 TAS 9 1 5 3 Tenth Island 1993 NT 8 1 6 1 Cobourg Peninsula 1934

(Source: Taronga.org.au)