Steven Fogarty, 24, was snorkelling in 2m water near the Windang boat ramp at 10.45am when he felt a nudge on his right calf.

"Something just grabbed me from behind," Mr Fogarty said.

"I thought someone was playing a joke."

Within seconds he realised something more serious was afoot and started punching out at the creature attacking him.

"All I could see was a white cloud with all bubbles around it," he recalled.

"I started screaming and I got on my back and started back-pedalling.

"I was in the water a good couple of minutes. A couple of people drove past in a boat. One bloke just went straight past me, just looking at me. I yelled 'help, help, help'.

"A fella on the bridge waved his hand."

At that point, Mr Fogarty, a waterfront worker, said he wasn't in excruciating pain.

"I was scared more than anything," he said. "I was still in a metre of water."

It was then that fisherman Pete Holm dragged him on board his boat and transported him to shore.

"There was blood all over the boat," Mr Fogarty said. "I had a quick look and made sure both legs were there."

Mr Fogarty was treated at the scene by paramedics before being transferred to Shellharbour Hospital.

District manager for Illawarra Ambulance Service, Terry Morrow, described the injuries.

"There were approximately 30-40 teeth marks around his right calf that appeared to be from the jaws of a shark.

"They were clean, simple cuts, like little razor blade cuts," he said.

Mr Morrow said fishing authorities had reported seeing bull sharks in the entrance of the lake over the past few days.

At Shellharbour Hospital, Mr Fogarty received stitches to his right leg before being released yesterday afternoon.

Speaking outside the hospital, he had advice for other lake users.

"Don't swim on your own," he said. "I've been told a thousand times.