Paramedics say a swimmer is "extremely lucky" to be alive after a shark attack near Sydney's Manly Beach left him with a deep gash to his leg.

Key points: Peter Shultz is an experienced ocean swimmer, who has previously conquered the English Channel

Peter Shultz is an experienced ocean swimmer, who has previously conquered the English Channel He sustained a puncture wound to his back and a "deep gash" to his leg in the attack

He sustained a puncture wound to his back and a "deep gash" to his leg in the attack The Department of Primary Industries is working to determine the shark's size and species

According to witnesses, Peter Schultz sustained the leg injury, a puncture wound to his torso and cuts to his back and stomach while he was swimming with a group at Shelly Beach before sunrise today.

The well-known ocean swimmer, who is part of the Manly-based Bold and Beautiful swimming club, was taken to Royal North Shore Hospital for treatment.

Danny Lucas was also swimming at the beach this morning and told ABC Radio Sydney there was initial confusion about the attack.

"We were about three quarters of the way to Shelly Beach when our group stopped and said someone was calling out," he said.

"We couldn't really see anything.

"But then we realised our friend had managed to actually swim to shore and was yelling for us to get out because he had been bitten by a shark.

"He had lacerations to the stomach and to the back and then a deep gash down his left leg."

Experienced ocean swimmer Peter Schultz in action. ( Supplied )

Mr Lucas said people called an ambulance after Mr Schultz got out of the water.

"There was a reasonable amount of blood on the footpath and there were a number of passers-by who were looking after him and calling the ambulance," he said.

"He [Mr Schultz] was very lucid and kept a good sense of humour, we were chatting to him."

NSW Ambulance acting inspector said Mr Schultz's injuries were not life threatening.

"He is extremely lucky," he said.

"We know this could have been a lot more serious than it was."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 21 seconds 21 s Paramedic says man attacked by shark was 'extremely lucky'

The NSW Department of Planning, Industry and Environment (DPI) said after assessing the wounds, scientists believed it was a grey nurse shark that attacked Mr Shultz.

However they could not determine the length of the shark.

The DPI described the species, which is critically endangered, as "generally harmless".

It is an offence to harass, harm or threaten grey nurse sharks.

NSW Police said no shark had been spotted in the area.

Mr Lucas said Mr Schultz did not see the shark.

"He just said he felt it, then waited a few seconds to see if it was going to come back," he said.

"We swam a bit more briskly than we normally would."