A father has been killed in a horrific shark attack on Sydney’s northern beaches.

The 57-year-old man had gone surfing with a group of friends at Long Reef Beach on Saturday when he was attacked about 10am.

WATCH THE VIDEO ABOVE: Police speak after surfer killed by shark on Sydney’s Northern Beaches

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Both the surfer and his board disappeared under the water, NSW Police Inspector Stuart Thomson said, with the man suffering “catastrophic injuries” including the loss of several limbs.

“His (friends) managed to make it back to the beach safely and a short time later his body was found floating in the surf and a couple of other people went out and recovered it,” Thomson said.

“Attempts to save his life were unsuccessful.”

The man was an experienced surfer who leaves behind a wife and young daughter, Thomson said.

“Tomorrow being Father’s Day is particularly … tragic.

“Our heart goes out to them.”

The surfer had only been in the water about half an hour when he was attacked and was about 100m from the shore.

The beaches remain closed until further notice. Credit: 7NEWS

The Westpac Lifesaver Rescue Helicopter searching for the shark. Credit: 7NEWS

His board was broken in half, with the pieces to be examined to help determine what kind of shark was responsible.

There were a number of people who witnessed the attack including the victim’s friends who were nearby at the time and other surfers who believed what they saw was a shark.

The NSW Department Of Primary Industries is taking the lead in investigating the incident.

“It’s a terrible, terrible thing and it’s one of those freak incidents that happens very very rarely,” Thomson said.

“Obviously there was a lot of people down at the beach at the time, its a beautiful day here at Dee Why, its a great tragedy.”

A number of police boats and jet skis have been out on the water looking for the shark, as well as the Westpac Lifesaver Rescue Helicopter.

Beaches between Manly to Narrabeen have been closed until further notice.

Surf Life Saving said Dee Why beach would likely be closed for up to three days with other beaches in the region expected to stay shut for at least 24 hours.