A 13-year-old surfer had her left arm bitten off by a shark on Kauai's North Shore, in Hawaii, and her best friend's father was credited with saving her life by using a surf leash as a tourniquet.

Bethany Hamilton, who has competed in national surfing contests and was expected to turn professional, was surfing half-a-kilometre off Makua Beach near Haena, in an area known as Tunnels, when the attack occurred on Friday morning, said Cyndi Ozaki, spokesperson for Kauai County.

Her arm was not immediately found, Ozaki said.

The Hamilton family said she is "expected to recover". She was taken to Wilcox Memorial Hospital, which released a statement that she lost her left arm just below the shoulder.

Bethany was surfing with her best friend, Alana Blanchard, also 13, when the attack occurred, the Hamilton family said.

Alana's father, Holt Blanchard, who was surfing with the girls, immediately applied a tourniquet to Bethany's arm using a surf leash, the cord that surfers use to attach the board to their ankle so they don't lose it.

Her mother, Cheri Hamilton, says Blanchard's quick action saved her daughter's life.

Witnesses told rescuers that Bethany was lying on her board in clear water when the shark attacked, according to Battalion Chief Bob Kaden. There was a single bite, and then the shark disappeared.

Bethany is a competitive surfer who already has secured sponsorships and is expected to turn professional, according to the Hanalei Surf Online website.

The size of the bite was estimated to be 40cm long and 20cm wide, which suggests the shark was 3,5 to 4,5 metres long, Kaden said.

Randy Honebrink, spokesperson for the state Shark Task Force, said it is possible that it was a tiger shark.

"That's the most likely culprit," he said. "They're the most likely to be capable of inflicting that type of injury."

The last shark attack this year occurred on October 5 when a 41-year-old woman was bitten while swimming near Kihei on Maui, Honebrink said.

The last shark attack on Kauai was in March 2002, when 17-year-old Hoku Aki lost a leg when a shark attacked him while he was bodyboarding in murky waters at Brennecke's Beach near Poipu.

Bethany's attack was the fourth this year.

Honebrink said Hawaii averages three or four shark attacks a year.

The largest number of attacks - eight - occurred in 2002. In 2001 there were only three attacks, two in 2000, five in 1999 and none in 1998, Honebrink said.

Meanwhile, in Sydney an Australian man is recovering in hospital after being mauled by a shark at Seal Rocks north of Sydney.

The man was swimming in half a metre of water when he was attacked. He was treated at the scene before making his own way to Taree Base Hospital. - Sapa-

AP-DPA