Beaches around La Perouse have been closed on Saturday after a woman suffered deep cuts to her leg in a suspected great white shark attack at dusk on Friday.

The woman, aged in her 50s, was swimming about 20 metres offshore at Congwong Beach in Botany Bay National Park about 7pm when she was bitten.

The woman suffered deep lacerations and puncture marks to her left leg and foot, but managed to make her way towards the shore.

The woman, who was bleeding heavily from her wounds, was helped onto the sand by a number of nearby fishermen, who gave her first aid and called emergency services.