Two surfers from the North-West had gone to Nettley Bay for the day and were in shallow water when the shark lunged at one of the surfer's boards, Surfing Tasmania executive officer Shayne Clark said.

The surfers thought they had spotted a fin further out earlier that day, Mr Clark said.

"The surfboard bruised and creased from the hit of the shark, it knocked him off the board," he said.

"If you've seen the Mick Fanning incident, I can only assume it's something like that."

The surfer was not injured and the shark quickly disappeared, Mr Clark said.

The species and size of the shark is unknown.

"This is just one of those unfortunate situations," he said.

"The shark has been probably chasing salmon and it's come upon these two bigger things in the water and given them a nudge.

"He [the surfer] said that there was some thrashing about with him in the water with the shark still there as he tried to get out."

Mr Clark said previous posts by Surfing Tasmania of reported sightings had gone "viral".

"In Tasmania, our protocol is we advise all surfers to keep out of the water that day," he said. "Sharks are often moving through, they're not necessarily staying in the one spot, and you're extra vigilant if you go back in the water over those few days."