An Australian school owner has defended his failure to warn his class that a large shark was swimming close to teenagers on a popular New South Wales beach.

A photograph captured the moment the shark, believed to be a nearly 10-foot great white, surfaced at Blueys Beach on the Mid-North Coast, near Myall Lake on Sunday.

Silvio Rodriguez was photographing other surfers near the surf school students about midday when he noticed a large shadow.

"At first I thought it was weed, but I kept an eye on it then I saw the massive fin come up," Rodriguez, 31, said. "It was only maybe 30 meters away from the nearest learner surfer."

Former professional surfer Gary Hughes, who runs Gary Hughes Surface School of Surf and Surf Consultancy, said he was aware of the shark and had been monitoring it.

"I did not tell the students because they just would have worried," Hughes, 50, said. "It was just having a cruise, it wasn't showing attack behaviour."

Elsewhere in the state, Wooloongong Council Monday said it would erect a shark warning sign near the entrance to Lake Illawarra, where a man was attacked on Monday morning, while surfers were warned Monday afternoon of a great white seen by aerial patrol near Fairy Bower at Manly.

Click here to read more on this story and to see the photo at News.com.au.