Simon Nellist, 35, had been training for a charity ocean swim when he was killed at 4.30 pm AEDT (5.30 am GMT) at Buchan Point, near Sydney’s Little Bay.
Emergency services did not name the victim when the tragic incident was reported on Wednesday.
Efforts to save Mr Nellist were in vain, and he died as a result of “catastrophic injuries”.
A spokesperson for the New South Wales ambulance service said: “Unfortunately this person had suffered catastrophic injuries and there was nothing paramedics could do.”
A police statement added authorities had examined the scene, and “located human remains in the water”.
This is believed to be the first death from an unprovoked shark attack in the city for nearly 60 years.
Kris Linto, a fisherman who witnessed the attack, told 9News he estimated the shark to be around 4.5 metres long.
He said: “The shark came and attacked them vertically.
“We heard a yell and then turned around.
“[The splash] looked like a car just landed in the water.”
Another witness recalled how he saw a man in a wetsuit swimming across the bay, before being dragged underneath the surface by a “large” shark.
The man told ABC News: “When he went down there were so many splashes.
“It was terrible. I am shaking.
“I keep vomiting. It’s very, very upsetting.”.
He added: “He just went down for a swim, enjoying the day, but that shark took his life.”
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