Chris Noble, a science teacher at North Sydney Girls High, was surfing off the coast of La Perouse near the site of a former fortress, Bare Island, when he spotted the boat in distress.

The boat capsized after it rode up the crest of a big wave and flipped over, Mr Noble said.

"I was looking at it thinking, 'Geez, there is a chance this could go over here'," he said.

Mr Noble was the only person in the area that could have responded quickly enough to rescue the people in the boat, Sergeant John Cox, from Marine Area Command, said.

It was also the first time the experienced surfer had taken on the break off Bare Island.

The crew of the small runabout boat was attempting to enter Botany Bay but ignored clear warnings not to go through an area close to the reef, where the boat capsized.

A number of boats have been flipped over in the same spot, Sergeant Cox said.

"There were also several big swell warnings issued."

Mr Noble paddled out 100 metres to the capsized boat and found two people who had been thrown overboard.

"I wasn’t freaking out, I just did what was logical at the time," he said.

"One of them was hanging onto the hull of the boat. He was screaming, 'help my friend he can't swim'."

Mr Noble said the other man was yelling and frantically flailing around in the water.

He got both men onto his surfboard to keep them afloat and then dove into the depths of the submerged cabin.

Inside, he found a 27-year-old fisherman desperately trying to escape the cabin, which was rapidly filling with water.

There was very little visibility in the cabin,  Mr Noble said.

"I just had to reach out and touch him to let him know I was there."

Mr Noble removed the obstacles and debris that were blocking the escape route and brought the man to the surface. He was also unable to swim.

The  Westpac Lifesaver Helicopter, police boats and even a lifesaver on a jet ski from Maroubra Beach joined the rescue.

But police believe that without the actions of Mr Noble the incident could have ended far more tragically.

"He did what every good Aussie should do,"  Sergeant Cox said.

"You've got to take your hat off and say well done."

The fishermen have been treated and discharged from hospital.

Mr Noble is modest about the courage it took to rescue the stricken fishermen.

"I was just doing the right thing. I was in the right place at the right time."

Sergeant Cox said that the NSW Police are recommending Mr Noble for a bravery award.