A father has recounted the moment he feared the worst just 30 seconds into a surf ski race off Seacliff Beach, in Adelaide's south.
Murray Drummond said he saw the athletes scatter in the water and went into a panic when he could not see where his son was among the other competitors who were coming back to shore.
"It was quite horrific," he said.
"The first thing that my wife and I saw was this surf ski lying across the IRB (inflatable rescue boat) with this massive great hole in it and we couldn't see Nathaniel in the boat itself.
"Even thinking about it now, it's making my heart race a bit.
"My legs went numb and all those sorts of things that happen when you go into a bit of shock."
Incredibly, Nathaniel escaped the attack uninjured.
The shark, which is estimated to have been up to three metres long, attacked the front of the 19-year-old's surf ski, sending him flying a metre into the air.
Mr Drummond said the rush of emotions he felt when he saw that his son was not injured was indescribable.
"I just ran over to him and gave him a big hug, you don't realise how much you absolutely love your kids until something like that happens," he said.
"I've got a beautiful son, he's my best buddy in terms of we do stuff together, we do a lot of sport, we paddle together, it's just incredible and I don't have to explain that to any other parent," he said.
Mr Drummond said the first thing he said to his son was "I love you" and "I'm proud of you".
"Because I knew what he would have done out there to stay alive," he said.
He said Nathaniel, who has been part of Surf Life Saving SA since he was six years old, went into "survival mode" as soon as he heard a thud under his ski.
"He still had his leg rope attached to his ski which you have to have in these ocean races, he quickly ripped that off," he said.
"All of the surf skiers banded together, they put their skis together because they are quite unstable craft.
"He swam over, he threw his leg over the top of the two skis while other surf skiers came towards to him ... it's a real kind of community and we are really grateful for all of the people that assisted."
One of the shark's teeth was found embedded into the surf ski.
Research scientist at Flinders University Lauren Meyer said they are running tests to determine the species of shark.
"We were able to take some samples from the board and look at which species actually bit it using some genetic tools, we will also verify that with the shape and size of the tooth," she said.
"It's likely that it was a white shark but until we get genetic results back, it's difficult to say."
Dr Meyer said there had only been 33 incidences in South Australia of sharks biting boats and boards in shallow waters since records began in 1873.
"It's exceptionally uncommon to have this type of incident in South Australia," she said.
Surf Life Saving SA's emergency and operations manager Daniel Willetts said there had not been any further shark sightings reported in the area.
"It was just an incident that occurred without any warning," he said.
"We know that happens, sharks get seen from time to time throughout this coastline, particularly at this time of year."
Mr Willetts said counselling services were being offered to those impacted by the attack.
"There's no doubt he is a lucky lad," he said.
He said it was important for beachgoers to be alert, but not alarmed.
"We know these things can happen, it's a risk we take when we enter the water," he said.
"When you are undertaking some aquatic activities, please do that in company of other people."
"Particularly we ask that you do it in front of a life-saving service so that if something does happen, you've got resources there to help you and respond and come to your aid."
