The man, in his 50s, had been surfing off Forster, 220km north of Sydney, when he was attacked late in the morning, a police statement said.
A surfer has died after being bitten by a shark at a beach north of Sydney.
Paramedics were called on Tuesday morning to Tuncurry Beach, about three hours' drive from Australia's most populous city, following reports of the attack.
"Despite the best efforts of paramedics and bystanders at the scene, the man could not be resuscitated," New South Wales Ambulance tweeted.
The man, believed to be in his 50s, went into cardiac arrest after being bitten on his thigh, a spokesman told AFP. Police said he was surfing when the attack occurred.
The New South Wales state government's SharkSmart organisation said beaches in the area had been closed until further notice following the incident.
It was the first fatal shark attack recorded in Australian waters this year, according to the Taronga Conservation Society Australia.
There were 26 maulings in 2020, eight of them fatal, data from the government agency shows.