A person then yells: "He's gone ... no he's swimming, he's swimming, come on mate chopper's coming back. Chopper's coming back."
With the board firmly stuck in the shark's mouth, Longgrass made a dash for the beach. The 41-year-old was lucky to escape with deep bite mark on his right leg.
The footage shows people running to the water's edge to help.
One says to him: "You're a lucky man. Oh my god."
Speaking after the attack, Longgrass said he slapped the water in a bid to scare off the shark.
"It was just heading straight for me, beelining it straight at me," he told Seven News Perth.
"I went to slap the water, by then he made a sudden acceleration and just nailed the board.
"It wasn't looking at me it was looking at the board."
The footage emerges as two of the world's top surfers have delivered a devastating assessment of the risk of a shark attack in Margaret River, saying they do not feel safe going in the water.
Brazilian surfing heavyweight Gabriel Medina told his 6 million Instagram followers that he did not feel safe training or competing in Margaret River and that he wanted to leave his opinion "before it's too late".
"Today they had two shark attacks on a beach close to where we're competing," Medina wrote.
"I do not feel safe training and competing in this kind of place, anytime anything can happen to one of us. Hope not. Leaving my opinion before it's too late!"
Medina's compatriot Italo Ferreira also took to social media to express similar misgivings.
"Two shark attacks in less than 24 hours here in Australia, just a few kilometres from where the event is being held," he said on Instagram. "Don't you think that's dangerous?
"Is the safety of athletes not a priority?
"I don't feel comfortable training and competing in places like this."
The international event was called off on Tuesday due to poor conditions, but organisers also warned competitors not to surf in the area.
"We have actioned our well-established safety protocols and are gathering all the latest information to determine the next steps," the World Surf League said. "We will continue to liaise with all involved, most importantly the surfers. Their safety remains paramount."
A political battle is now waging in WA over the response to the attacks.
The state's Liberal opposition says sharks are making the state appear dangerous to the world and more needs to be done to prevent attacks.
Opposition leader Mike Nahan says money should not be a factor and wants clever buoys, SMART drumlines, drones and the reopening of the southwest shark fishery.
"We are showing the world that we are a dangerous place because of sharks — well, fix it," he told reporters.