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.

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.

Sutherland Shire Council has now warned swimmers not to enter the water around Kurnell and for dog owners to keep their pets away from the water following the incident. Signs have been erected along the beachfront warning of a shark sighting.

Nigel, who did not want his surname used, said he had owned Molly for two years after rescuing her from the pound.

He and his wife had been at Bonna Point Reserve with Molly for about 30 minutes on Sunday afternoon and were just about to leave the beach when the attack occurred.

"We were just throwing the stick in the water. The tide was really far out and there was a bit of a drop off [where the water became deeper]," he said.

Nigel said he was right on the shoreline and Molly was about five metres off-shore when the shark took her.

"It was very quick. It just took her under the water," he said. They did not see Molly again, and could not retrieve her body.

Nigel said there were no other dogs in the immediate area, but they alerted other dog owners about what had happened as they returned up the beach.

"We just freaked out and we told a few people that we saw on the way back. I also rang the council to let them know," he said.

Apart from the shock of losing his beloved pet, Nigel said swimmers should also be aware.

"The shark was around 3.5-metres long, which is easily big enough to take a human," he said.

A Sutherland Shire Council spokeswoman said council lifeguards went to Kurnell on Monday and warned swimmers and pet owners to avoid the water.

"NSW Department of Primary Industries advises people to avoid swimming or surfing when it is dark or during twilight hours and encourages the use of the SharkSmart app, which has safety tips for swimmers, surfers, divers, snorkelers and spearfishers," the council said in a statement.

The incident comes after several sightings of sharks close to swimmers and kite surfers in Botany Bay this month.

On February 9, a kite-surfer was filmed just metres from a shark close to the shore at Brighton Le Sands, as people on the beach waved to him and tried to alert him to the shark's presence.

Several days later, two swimmers were photographed running across shallow water near Sydney Airport as a shark's dorsal fin surfaced not far from them.

Around the same time, a juvenile great white shark was swimming in Alexandra Canal, a tributary of the Cooks River close to Sydney Airport.

smh.com.au