The owner of a dog that was killed by a shark at a popular off-leash beach in Sydney's south said he was traumatised after watching his pet being pulled under the water just metres from the shore.

The American Staffordshire Terrier, a rescue dog named Molly, was fetching a stick from the water off Bonna Point Reserve in Kurnell about 3pm on Sunday when the 3.5-metre shark seized her.

Bonna Point at Kurnell, pictured in 2016, is a popular off-leash area. Credit:Jeff Darmanin

Molly's owner, Nigel, said there was nothing he or his wife could do to save Molly from the shark, which was believed to be a bull shark.

"We were pretty traumatised, it was horrific," said Nigel, from Sydney's south.