A woman has suffered severe injuries after being flung from a flying fox at a family gathering on Sydney's northern beaches.

Northern Beaches Police said the Curl Curl woman, 38, got on the apparatus at a property on Morgan Road, Belrose, north of Frenchs Forest, on Sunday afternoon after seeing children enjoying the same ride.

Six ambulance crews attended the scene. Credit:Katherine Griffiths

Two braking devices were installed on the ride, which had been strung up between two trees and was about 40 metres long, police said. However, they did not slow the woman's passage.

"She was flung at the end of the ride into the tree," police said.

Police said the woman is suspected to have suffered a severed spinal cord, head injuries, broken ribs and a punctured lung.

A NSW Ambulance spokesman said the woman had suffered head and chest injuries.

Six ambulance crews and a rescue helicopter went to the property. The woman was flown to Royal North Shore Hospital in a critical condition, the NSW Ambulance spokesman said.