Emergency crews were dispatched to Long Reef Beach shortly after 10am on Saturday, responding to reports a man had suffered critical injuries, police said.
He was retrieved from the surf and brought to shore but died at the scene.
Police have closed the beach and will liaise with wildlife experts to determine the species of shark involved.
Two sections of a surfboard have been recovered and taken for examination.
Beaches from Manly to Narrabeen have been closed pending further advice.
The identity of the man is to be confirmed.
Witnesses say the victim was brought ashore by two fellow surfers and that distraught family members rushed to the scene.
AAP has been told injuries to the man's limbs were extensive.
Police at Dee Why have scheduled a press conference on the tragedy for 1.30pm. Â
The attack is thought to be the first in NSW this year.
The last fatal 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.
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