The alarm was raised by crew aboard the Westpac Rescue Helicopter who spotted the three-metre sharks about 11.20am in the calm, clear water off Bennetts beach.

Crew chief Pete Cummings said one was "pretty close" behind a group of surfers, the same distance from shore as nearby swimmers.

"There were about 100 or so people having a swim," Mr Cummings said.

"[The sharks] didn't look like they were chasing fish, they were just cruising."

Patricia Beatty, who was at Hawks Nest Surf Club when swimmers were being asked to leave the water, said it was the second time she'd waited anxiously for her 14-year-old daughter Laura to get ashore safely, after a similar incident early last week.

"It was quite close, it was very scary," Mrs Beatty said.

Beachgoers were put on alert on the Central Coast last week after five sharks were spotted chasing fish along the shoreline less than 12 hours after a surfer was attacked.

The sharks, including two about four metres long, were spotted close to where the surfer was bitten on the arm during a twilight surf at North Avoca Beach.

Shark populations along the Hunter and Central Coast have been tipped to boom due to nutrient-rich water, plentiful baitfish and warmer currents.

But it may be dangerous surf conditions that keep swimmers out of the water today and again tomorrow.

Surf Life Saving Australia has advised swimmers, surfers, rock fishermen and boat skippers to take extreme caution as the dangerous conditions develop along the coast today from the Hunter through to the far South Coast.

A large southerly swell upwards of three metres is expected to develop, creating large waves at beaches, rock shelves and headlands, and river bar crossings.

Swimmers should only swim between the flags at patrolled beaches, Surf Life Saving NSW lifesaving manager Dean Storey said.

"People need to take extreme care when visiting the coastline in the coming few days," he said.