The 52-year-old man sustained significant wounds to his left leg and hand in the dawn attack at Evans Head, 550 kilometres north of Sydney, Detective Inspector Cameron Lindsay said.
The victim, whose name has not been released, was taken by ambulance to Lismore Base Hospital in serious but stable condition, police said.
Recreational diver Damian Johnson, 46, became the second victim of a fatal shark attack in Australia this year when he was mauled on Saturday off the island state of Tasmania.
"They then proceeded to try to paddle in from the ocean and ... It appears the shark has then attacked the 52-year-old and has bitten him on the left-hand side," Lindsay told reporters at the tourist town of Ballina, 40 kilometres north of Evans Head.
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"He actually fought off the shark, we're told, and has made it to shore with the help of that other surfer and passers-by," he said.
Beach walkers saved the victim's life by using surfboard leg ropes as tourniquets to slow the bleeding before paramedics arrived, Lindsay said.
Beaches around Evans Heads were closed for 24 hours following the attack. Ballina was the scene of two recent shark attacks in which 32-year-old surfer Matt Lee was critically injured on July 2 and Japanese tourist Tadashi Nakahara, 41, was mauled to death while surfing in February. Ballina is 1,600 kilometers north of the scene of last week's fatal attack.
Sharks are common off Australia's beaches, but fatal attacks are rare. The country has averaged fewer than two deadly attacks per year in recent decades.