SYDNEY — A woman has died and a man has been seriously injured after they were attacked by a shark while swimming at a remote beach in the Australian state of New South Wales.

Police said the woman, believed to be aged in her 20s, died at the scene, while the man, also believed to be in his 20s, was airlifted to a hospital in Newcastle in critical condition.

Paramedics had been called to Kylies beach, in Crowdy Bay National Park about 300km north of Sydney, early on Thursday morning local time, police said.

Authorities praised a bystander who put a makeshift tourniquet on the man's leg before the paramedics arrived, which "potentially saved his life".

"The courage from some bystanders is amazing in this situation," NSW Ambulance Superintendent Josh Smyth told reporters.

"To put yourself out there is heroic and obviously it did give us time to get to that male patient."

The beach has now been closed and police were liaising with experts from the Department of Primary Industries to determine the species of shark involved.

"Smart" drumlines — which lure sharks with baited hooks so they can be tagged and re-released — were also being deployed.

Surf Life Saving NSW said surrounding beaches would also be closed for 24 hours while drones surveilled the area.

"This is a terrible tragedy and our deepest condolences go to the families of the woman and man involved," Surf Life Saving NSW chief executive Steve Pearce said.

"For now, please remain clear of the water at beaches in the vicinity and follow the direction of lifeguards."

This is the fifth fatal shark attack in Australia this year. The most recent was less than three months ago, when a man was killed in a rare shark attack in Sydney's northern beaches. — BBC