A two-year-old girl has died after being hit by a reversing car in the south-west Sydney suburb of Campsie.

The Kia vehicle, driven by a 43-year-old female driver, was being reversed in a car park at the back of a mechanics on Beamish Street when the accident happened.

"Paramedics arrived on scene within three minutes and quickly identified that the two-year-old girl was in cardiac arrest with life-threatening head, chest and neck injuries," NSW Ambulance chief inspector John Brotherhood said.

The girl was taken to Sydney Children's Hospital at Randwick, where she died.

Police are on site and will prepare a report for the coroner. ( ABC News: Kathleen Calderwood )

"Our heart and our sincere condolences at this time, close to Christmas, goes out to everyone involved in this," Chief Inspector Brotherhood said.

"Police were excellent in their assistance in providing traffic control and moving the public out of the way in the M5 tunnel so that we could get there in half the time that it would normally take because we needed to get this child to the hospital as soon as possible.

"They do this every day for us, it just doesn't usually hit the news."

NSW police have begun an investigation and the driver was taken to hospital for mandatory urine and blood tests.

"NSW Ambulance does sincerely apologise to the public for the chaos we caused for them on such a busy day but we did need to get the child to the hospital as soon as possible," Chief Inspector Brotherhood said.