Swimmers and pet owners are being warned stay away from a south Sydney beach after a dog was taken by a shark while swimming off shore on the weekend.

Authorities from Sutherland Shire Council say the pet dog was in waters at the off-leash beach at Bonna Point in Kurnell on Sunday evening when he was attacked.

Lifeguards were patrolling Kurnell beaches yesterday to warn people to keep out of the water, the council said.

Signs have also been installed to alert the public to the shark sighting.

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"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, snorkellers and spearfishers," the council said in a statement.

- AAP