A search-and-rescue helicopter is sweeping Sydney's northern beaches for any signs of a shark that is believed to have knocked a surfer of his board at Dee Why.
The Westpac Life Saver Rescue Helicopter will fly over Dee Why and surrounding beaches on Monday morning and report back to lifeguards before they open the beaches, helicopter air crew member Trevor Cracknell said.
"We just want to have a really good look, just in case that shark is still hanging around," he said.
An off-duty Warringah Council lifeguard, Danny Sheather, was knocked off his board while surfing at Dee Why about 12.45pm on Sunday.
Mr Sheather did not see what hit him but noticed a 30-centimetre chunk out of the bottom of his board when he returned to shore.
"It looked like something had come up and rammed him from underneath," Steve Downman, a Warringah Council lifeguard supervisor, said.
Dee Why and Long Reef beaches were immediately closed.
Mr Cracknell said the rescue helicopter was sent up to search for the shark on Sunday afternoon, and soon spotted a 2.5-metre white shark at the south end of Dee Why.
The helicopter took off from Sydney just before 8.30am on Monday to sweep the northern beaches again.
"If we see something out there we can get the beaches closed very quickly because we can talk directly to lifeguards," he said.
Sunday's incident was the second involving a shark on NSW beaches in a week.
Luke Allan, 29, lost a finger to a shark near Port Macquarie on Friday but said he plans to surf again. "Absolutely, the first chance I get … I don't know of anyone that's been attacked twice."