Teenage girl seriously injured after falling off a boat on Sydney lake

A teenage girl suffered serious head injuries after falling off a boat on Narrabeen Lake on Sydney's Northern Beaches.

The 15-year-old was out with friends when the accident happened about 1.20pm. She fell off the boat and was then hit in the head with a propeller.

Her friends brought her to shore on a paddleboard and she was treated at a nearby Melwood Oval by paramedics.

CareFlight's specialist doctor and NSW Ambulance critical care paramedic conducted a clinical assessment on the patient, including an ultrasound, CareFlight said.

They administered pain relief and intubated the teenager and placed her in an induced coma to stabilise her condition.

She was then airlifted to The Children's Hospital at Westmead where she remains in a critical condition.

She suffered head and suspected spinal injuries.

Officers from Northern Beaches Police Area Command and the Marine Area Command are also investigating the circumstances surrounding the incident.