A man has died after being attacked by a shark in a "freak incident" that will 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 beaches on Saturday morning.

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

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

Superintendent John Duncan told reporters the attack took place 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.

Pieces of a surfboar

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