A three-year-old child has died after being locked in a car at a property in Sydney's north-west on Christmas eve.

The young girl's family found her unconscious on the back seat of their white sedan in Cameo Circuit, Glenwood, about 4.45pm.

When police arrived, family were performing CPR inside the home, with police taking over until the arrival of NSW Ambulance paramedics.

A NSW Ambulance spokeswoman said that, when paramedics arrived, the girl was in cardiac arrest.