A man in his 50s has died after being attacked by a shark in Byron Bay on the New South Wales far north coast, police say.

According to The Northern Star, paramedics were called to Lawson St behind Clarkes Beach to treat a man with a potentially fatal shark bite.

Earlier reports suggested the victim was a surfer.

Witnesses called triple-0 at 10.42am on Tuesday saying a man had been bitten by a shark at the southern end of the beach, off Lawson Street.

Paramedics who rushed to the scene were unable to save the man's life.

"He did have a serious leg injury and as a result has unfortunately died on the beach after being treated by a doctor and paramedics," the officer said.

Police understand that the man was attempting to swim from The Pass to Byron Bay's Main Beach, a popular swimming route, when he was attacked.

Bystanders noticed a man floating in the surf, paddled out towards him and dragged him to the beach. Witnesses said that part of the man's leg was missing.

Shortly after the attack, a shark, estimated to be two metres long, was spotted near the site where the victim died by a Channel 7 helicopter.

A photograph of a shark spotted nearby shortly after the fatal attack. Photo: Channel 7/Fairfax

This is the fourth fatal shark attack to occur in the Northern Rivers region since 1982. Previous attacks have occurred at Tallows Beach (1982), Julian Rocks (1993) and Lighthouse Beach, Ballina (2008).

Surf Life Saving NSW spokeswoman Donna Wishart said Byron Bay beaches were closed from Belongil to Tallows Beach, south of Cape Byron for the next 24 hours and has advised swimmers and surfers not to enter the water.

An image from the scene showing a crowd growing at the scene.

More to come as details come to hand.