Detectives are investigating the circumstances surrounding the death of a 16-year-old girl in Sydney's Northern Beaches, with early indications suggesting she may have died during a consensual sex act.

Emergency services were called to a residence in Elanora Heights around 5:45 p.m. on Sunday following reports that the girl was experiencing a seizure. She was found unresponsive at the scene and was transported by air ambulance to Royal North Shore Hospital in critical condition. New South Wales Police confirmed her death on Tuesday evening.

A young male, who was known to the girl, is reportedly cooperating with the ongoing police investigation. Authorities believe the girl may have lost consciousness after being choked during the sexual encounter.

"Officers attached to Northern Beaches Police Area Command commenced an investigation into the incident. Inquiries are ongoing," police said in a statement.