'Tragedy': Dad fatally mauled by 'obviously huge' shark

A man has died after being attacked by a shark in a "freak incident" likely to close beaches for days as authorities investigate.

The 57-year-old was mauled by a large shark at Long Reef Beach on Sydney's northern peninsula on Saturday morning.

He was retrieved from the surf and brought to shore but died at the scene.

Police closed the beach and are liaising with wildlife experts to determine the species involved.

Superintendent John Duncan told reporters the attack happened about 100m offshore.

"It's a terrible, terrible thing," he said.

"It's one of those freak incidents that happens very, very rarely ... a great tragedy."

The man leaves behind a wife and young daughter, Supt Duncan said.

"Tomorrow being father's day, it's particularly tragic," he said.

The man "lost a number of limbs," Supt Duncan said.

Pieces of a surfboard were recovered and taken for examination.

Beaches from Manly to Narrabeen will remain closed at least until Sunday.

"We anticipate Dee Why will be closed for 48 to 72 hours," Surf Life Saving NSW duty officer Rod McGibbon told reporters.

NSW Premier Chris Minns called the attack an awful tragedy.

"My heart goes out to the victim's family and friends," he said in a statement.

"Shark attacks are rare but they leave a huge mark on everyone involved, particularly the close-knit surfing community."

Charles Prior, 33, heard about a shark as he was preparing to paddle out, after his wife came in from the water to take over caring for their baby.

"I thought, 'oh whatever, there are always sharks in the water ... then someone said 'don't go out, a guy's just been attacked'," he told AAP.

Other surfers began paddling toward a black shape in the water, which Mr Prior soon realised was the man's body.

He and other surfers helped bring the man through waist-deep water to the shore.

Another surfer described seeing a "massive white" shark, up to six metres in length, he said.

"It (the shark) is obviously huge, given it just removed two legs pretty well clean and his left hand."

A group believed to be friends and family of the victim gathered on the sand on Saturday afternoon.

Surf Life Saving NSW deployed a drone and jet skis to search for the shark.

"Our deepest condolences go to the family of the man involved in this terrible tragedy," chief executive Steve Pearce said in a statement.

A Department of Primary Industries spokesperson said the department was assisting the response.

"NSW government shark biologists will assess photographs of the incident to determine the species of shark involved," they said.

Northern Beaches Council issued a statement thanking the "many involved ... for their swift and professional response including members of the public ... first responders, lifeguards, SLS volunteers, police and paramedics".

The fatal attack, to be investigated by the coroner, is thought to be the first in NSW this year.

The last deadly attack in Sydney occurred in February 2022, when British diving instructor Simon Nellist was taken by a great white off Little Bay in the city's east.

On the northern beaches, the last known fatal attack was at North Narrabeen, 90 years ago.

Shark nets were installed at 51 beaches between Newcastle and Wollongong on Monday as part of a seasonal program.

The NSW government has announced a trial to reduce the number of shark nets, asking three councils including Northern Beaches Council to nominate a beach where nets could be removed.

Long Reef Beach does not have a net but nearby Dee Why Beach does.

Additional shark alert drumlines were deployed in the area on Saturday between Dee Why and Long Reef beaches.