Surfer finds himself standing on the thrashing shark off the coast of Oregon as he tries to catch a wave

Doug Niblack was trying to catch another wave before going to work when his longboard hit something hard as rock off the Oregon coast and he found himself standing on a thrashing great white shark.

Looking down, he could see a dorsal fin in front of his feet as he stood on what he described as 10 feet (three metres) of back as wide as his surfboard and as black as his wetsuit. A tail thrashed back and forth and the water churned around him.

"It was pretty terrifying just seeing the shape emerge out of nothing and just being under me," he told the Associated Press on Wednesday. "And the fin coming out of the water. It was just like the movies."

The several seconds Niblack spent on the back of the great white on Monday off Seaside, Oregon, was a rare encounter, but not unprecedented, according to Ralph Collier, president of the Shark Research Committee in Canoga Park, California, and director of the Global Shark Attack File in Princeton, New Jersey.

He said he had spoken to a woman who was kayaking off Catalina Island, California, in 2008 when a shark slammed her kayak from underneath and sent her flying into the air. She then landed on the back of the shark, Collier said. "At that point the shark started to swim out to sea, so she jumped off its back," Collier said.

Zach Vojtech of the US coastguard said officials did not officially log shark encounters, but he had learned about Niblack's ordeal from an off-duty member, Jake Marks, who was nearby when he was knocked from his board.

Marks said he never saw the shark, but saw Niblack suddenly stand up, with water churning around him. He joined Niblack in paddling as fast as he could for shore after seeing a large shape swimming between them just beneath the surface.

"I have no reason to doubt there was a shark out there," said Marks. "With the damage to his board, the way he was yelling and trembling afterwards – there is no other explanation for that."

Niblack estimated that he was standing on the shark for no more than three or four seconds. The dorsal fin caught his board and dragged him for about a metre by his ankle tether. "I'm just screaming bloody murder," he said. "I'm just yelling: 'Shark!' I thought for sure I was gone."

In six years of surfing, Niblack said he had seen sharks in the water, but never so close. He said he had been dreaming about sharks, but was planning to go back out to surf. When he does he will take a waterproof video camera his roommate gave him. He has also put a sticker on the bottom of his board to ward off sharks – a shark with a red circle and a slash over it.

"I'll definitely go back out," he said. "It's just the surf sucks right now. I'll wait until that gets better, then go back out."