Ambulance paramedics were deployed to the scene in Earlwood, about 10 kilometers southwest of central Sydney, but the girl was declared deceased at the scene.
A male occupant of the vehicle, believed to be related to the child, is assisting with inquiries at local police station.
Speaking at a late night press conference outside the childcare center, NSW Police Superintendent Christine McDonald said that police believe the child was left inside the vehicle for an extended period of time on a hot day.
The maximum temperature in Sydney exceeded 30 degrees Celsius on Tuesday.
