A scientist has ruled out claims a Sydney surfer was attacked by a great white shark this morning, pinpointing the attacker as the usually docile, bottom-dwelling wobbegong.

Paul Welsh, 46, was attacked while teaching his 10-year-old son to surf about 8am AEDT at the Mona Vale Basin on Sydney's Northern Beaches.

Mr Welsh said he had to cling to a rock to fight off the animal, which had latched onto his leg.

"I just felt something grab my leg and it tried to pull me under, as they do, and I just grabbed a rock so it wasn't going to pull me under," he said.

"I just kicked as hard as I could and it opened up its jaws and let me go. Obviously I wasn't the right food, so I live to fight another day.

"I didn't see it [but] my son said he saw it before, which he obviously didn't tell me and he waited for dad to get bitten.

"Rather me than him, because it probably would have taken his leg off."

By the time his wife had driven him to hospital, Mr Welsh had already sold his story to Channel Nine News.

Channel Seven later bought video footage of the aftermath of the attack.

Shark spotter Michael Brown told reporters he had witnessed the ordeal while swimming with his own son just 30 metres away.

Mr Brown, who has been pushing for greater protections against sharks, said the animal that bit Mr Welsh appeared to be a two-metre great white, by the look of its tail.

"[He] just had a big bite mark in his leg and blood just streaming out of it," he told the ABC.

"I'm quite shaken and I spend my life researching, following and working with sharks, but to be that close to an actual attack and to be faced with the fact that it could've been my child, it's definitely had a life-changing impact. There's no two ways about that."

The victim was released from hospital around midday after doctors found a tooth fragment in his leg.

Stirring shark fears

The beach was closed as a helicopter searched for the shark, but it was left to a scientist to find out the truth.

Tests by a biologist confirmed the fragment came from a 1.6-metre-long wobbegong, a shark not usually associated with aggression or attacks, unless it is stepped on.

Mr Brown has stirred fears about sharks in Sydney in the past.

After three attacks in the city last summer, he accused the NSW Government of ignoring warnings that shark numbers were on the rise.

Today he told reporters an increasing number of sharks were being spotted on the Northern Beaches.

But NSW Primary Industries Minister Steve Whan has questioned Mr Brown's integrity in light of this afternoon's revelation.

"It certainly wasn't a great white shark and I think you'd not have to know too much about sharks to actually know the difference," he said.

"A wobbegong shark is brown. It tends to dwell on the bottom, it doesn't come in from the top and really I think that's perhaps being generous to the panic of the situation.

"It doesn't fill you with confidence, does it?"

Mr Brown runs SurfWatch Australia, a small voluntary shark patrolling organisation that charges people at least $50 to come along for the helicopter ride.