Swimmers at Wollongong City Beach were asked to leave the water for about 10 minutes just before 11.30am, after spotters from the Australian Aerial Patrol plane sighted at least 12 sharks offshore.

Lifesavers in a dinghy chased one group of hammerheads out to sea at the southern end of the beach after the sighting.

But according to the aerial patrol team the sharks had returned by the time the plane flew back over an hour later.

Lifesavers monitored the area for the rest of the day.

There was also a smaller group of sharks at Windang and lifesavers were alerted, but Australian Aerial Patrol general manager Harry Mitchell said it was not unusual to see them in that area.

They were less than 100m offshore however, Mr Mitchell he said.

The sharks at Wollongong were most likely feeding near the sewer that empties into the ocean, which would attract small fish, he said.

At 1.50pm lifesavers asked swimmers at the south end of City Beach to again leave the water as the sharks had come closer to shore. But they kept their distance from the flagged swimming area further north.

Lifesaver Aiden Blair from City Beach said the shark bell had been rung as a precaution at the first spotting but lifesavers were comfortable letting swimmers back in the water after they had taken the dinghy out.

"We did a search and it was all fine for them to go back in," he said.

On Saturday, the Mercury revealed the Australian Aerial Patrol was sighting more sharks along the South Coast than ever before.

There are only about 13 reported attacks in Australia each year and few in NSW compared to other states, but Mr Mitchell said it was not uncommon to see sharks on most of his daily patrols