Sydney - Two popular Sydney beaches were closed on Sunday after a surfer told lifeguards that a shark had knocked him off his board and left a 30cm gash in the polyurethane.Dee Why and Long Reef beaches were shut after off-duty lifeguard Danny Sheather, 23, raised the shark alarm."I got a wave, paddled in and got up and I was going along and felt this almighty hit from the bottom," Sheather told local news agency AAP. "Then I turned over and there's this big chunk out of my board."Helicopters were sent out to shoo a shark out to sea."The shark was about 2.5 metres long and was inside the breaking waves close to shore," a helicopter spokesperson said. "It was swimming in circles between two rips right where the surfer was hit by something."Last week at Diamond Head Beach, 366km north of Sydney, a 29-year-old surfer had his finger bitten off by a shark and also sustained a large gash to the leg.Australia has recorded an average of one fatal shark attack a year in the 50 years that proper records have been kept.