They said a lifeguard on a surfboard rushed over when they heard screams and a shark alarm was sounded.
A NSW Ambulance spokesperson said they had multiple resources on route including a rescue helicopter.
She was being treated for serious leg and arm injuries and airlifted to hospital.
In January, three shark attacks happened at local Sydney beaches in a timeframe of just over 24 hours.
One of the attacks in Sydney Harbour where a 12-year-old boy was hospitalised with critical injuries, while another was a surfer in his 20s in critical condition after an attack at Manly beach.
An 11-year-old surfer was assisted, uninjured, off Dee Why Beach after a shark bit his surfboard.