A young girl is in a critical condition suffering from head injuries after being hit by a vehicle on Windsor Road in Sydney's Hills district.

The girl, who police say is about 12 years old, is breathing but unconscious, NSW Ambulance has confirmed.

A Careflight helicopter and four ambulance crews rushed to the girl's assistance after an emergency call came in at 4:35pm today.

The girl has been driven with the Careflight medical team to Westmead Children's Hospital, where her condition remains critical.

The male driver of the Toyota Hilux which struck the girl was uninjured in the collision. 

He has been taken to Westmead Hospital for mandatory blood and urine tests, police have confirmed.

The southbound lanes and all but one northbound lane of Windsor Road have now been reopened to traffic, but traffic remains heavy in both directions. 

Motorists are being advised to allow extra time driving through the area, with significant delays affecting the peak hour rush.

Due to heavy traffic, buses on the 615X, 618X and 628 routes are delayed by up to 20 minutes. 

More to come.

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