The experienced surfer, identified by Australian media as Mercury Psillakis, was mauled to death by a shark on Saturday off a popular beach in northern Sydney, police and rescuers say, leading to a string of beach closures.

The man, who left a wife and young daughter, lost "a number of limbs" and his surfboard was broken in two, police said.

It was the first fatal shark attack in Sydney since 2022, when 35-year-old British diving instructor Simon Nellist was killed off Little Bay.

The previous fatal attack in the city was in 1963.

Biologists with the New South Wales government were called in to assess the species of shark involved in the latest deadly encounter.

"NSW Government shark biologists have assessed photographs of the victim's surfboard and determined a White shark approximately 3.4-3.6 metres in length was likely responsible," a spokesperson for the state authorities said.

"The NSW Government's thoughts are with the victim, his family and the first responders."

State government authorities said they had deployed two extra smart drumlines -- anchored buoys with baited hooks that send an alert when a shark bites and also allow the animals to be tagged with trackers.

- Sharks 'all year round' -

The drum lines were set up off the adjoining Dee Why and Long Reef beaches where the shark struck, in addition to three already in place.

The two beaches remained closed on Sunday.

Surf lifesavers were also deploying drones and patrolling on jet skis to monitor for shark activity, officials said.

"Sharks are present in NSW waters all year round," the New South Wales government said.

"Sharks can be more prevalent in an area due to a number of factors including weather and ocean conditions as well as the availability of food in the area."

Ocean-loving Australians were advised to monitor a so-called SharkSmart app, which alerts swimmers and surfers in real time when a tagged shark nears popular beaches.

Australia's previous deadly shark attack was in March, when a surfer was taken off a remote beach in Western Australia.

There have been more than 1,280 shark incidents around Australia since 1791, of which more than 250 resulted in death, according to a database of the predators' encounters with humans.