Box Beach was closed to the public on Tuesday after a large whaler shark was spotted swimming close to at least one surfer by a surveillance helicopter.
The NSW Department of Primary Industries (DPI) aerial team on board the helicopter sighted the 2 metre long shark at the Shoal Bay beach shortly before noon.
Authorities were notified and the beach was evacuated.
"If the pilot spots a shark that poses a threat to surfers or swimmers, a call is immediately made the local surf lifesaving club," a NSW DPI spokesperson said.
"Water users will be made very aware if the helicopter spots a potentially dangerous shark.
"The helicopter will reduce its height to hover, a siren will be sounded, and beach authorities are notified."
It was not made clear by DPI whether the siren was sounded on Tuesday.
A helicopter flies along the coastline between Birubi and Crowdy Head once a day each day of the school holidays as part of the NSW Government's shark management plan.
Aerial surveillance for the winter school break began on July 6 and will wrap up on July 21.
"Aerial surveillance is used to monitor sharks that may be dangerous to swimmers and to record other marine wildlife that will help scientists understand factors that may influence shark movements and distribution," the DPI spokesperson said.
"Aerial shark surveillance from helicopters in metropolitan NSW coastal waters has been in action since 2009 as part of the Shark Meshing Program."
A photo posted to the NSW Shark Smart Twitter account by the DPI aerial team on Tuesday show the whaler shark swimming not far from a boardrider who appeared to be dressed in a wetsuit with flippers, in the clear Port Stephens water.
Shark sighting information is available via @NSWSharkSmart on Twitter.