A shark that took a 30-centimetre chunk out of a lifeguard's surfboard continues to roam the waters off a Sydney beach.

Dee Why and Long Reef beaches in Sydney's north remained closed on Sunday afternoon after off-duty Warringah Council lifeguard Danny Sheather was knocked off his board.

He was surfing about 100 metres off the south end of Dee Why beach about 11.50am (AEDT) on Sunday when the drama began.

"I got a wave, paddled in and got up and I was going along and felt this almighty hit from the bottom," Mr Sheather told AAP.

"And it kind of shouldered me forward off my board.

"Then I turned over and there's this big chunk out of my board."

Mr Sheather said he did not see the predator, but noticed a 30cm chunk, in the form of a semi-circle, missing from one side of his board.

He then paddled in and raised the alarm.

Dean Storey from Surf Life Saving NSW said rescue helicopters were keeping an eye on a 2.5m shark near Dee Why beach.

"As long as the shark is in the area the beaches will remain closed," Mr Storey told AAP.

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Shark scare closes two Sydney beaches