Nathan Shepherd underwent surgery on his arm after the fur seal leaped out of the water and bit him at Shelly Beach in Manly on Sunday afternoon.
"I thought it was amazing to be out with a seal. It was swimming closer to me. It swam quite fast -- it jumped out of the water and bit me on the upper arm," the 45-year-old told the Manly Daily newspaper.
"I tried to bring the surfboard down on top of it, and then to put the surfboard between me and the seal, and then I got back on the board and paddled to shore."
The wound on his right arm was "like a tooth mark. It's deep," Shepherd said, adding: "It was a freak occurrence."
Lifeguard Victoria O'Halloran said the seal "was incredibly aggressive... I've never seen anything like it".
An hour earlier the seal had knocked another surfer off his board and sunk its teeth into the board.
"It literally came from underneath me and knocked me right off the board with some pretty big force. I've got two big bite marks on there," surfer Tom Wallington told the Daily.
Pictures in the paper showed the bite marks and beachgoers gathering around the seal to take photos when it came ashore.
Shona Lorigan from marine wildlife rescue group ORRCA -- which was called to the beach -- said the creatures were wild animals with "very, very, very sharp teeth" and would defend themselves if threatened or provoked.
Under New South Wales state law, people should not be closer than 40 metres (131 feet) to an adult seal when it is on land, no closer than 10 metres in the water, and 80 metres from a pup.
"You never ever stand between a seal and their escape to the water. That also can cause some real issues," Lorigan told AFP.
The attack came a day after a shark crashed into a man's board while he was surfing near popular tourist spot Byron Bay, some 800 kilometres (500 miles) north of Sydney.
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