The 15-year-old boy, whose name was not disclosed, was surfing at Wooli Beach, near Grafton, around 370 miles north of Sydney, when the creature struck shortly before 2:30 p.m. local time (12:30 a.m ET), New South Wales police said in a statement.
"Witnesses have told police a shark attacked the teenager while he was surfing," the statement said.
"Several board-riders came to his assistance before the injured teen could be helped to shore," it said, adding that first aid was rendered for serious injuries to his legs.
Despite CPR efforts to revive him, the teenager died at the scene, the statement said. Beaches in the surrounding area had been closed, it added.
It is the second shark attack off the Australian coast in a week.
Last Saturday, a 20-year-old scuba diver who was spear-fishing died after they were attacked by one of the creatures off the coast of Australia's Queensland state.
A 57-year-old diver was also killed off Western Australia state in January, and a 60-year-old surfer was killed off Kingscliff in New South Wales state in June.
New South Wales police said officers and local authorities would start an investigation and report into the teenager's death.
The Associated Press contributed to this report.