Police are hunting for a motorbike rider who ran over a little girl’s head and chest, leaving her in a serious condition in Sydney.

Emergency services were called to Haydon Park on Copperfield Dr in Rosemeadow after the girl was struck by what police believe was an unregistered dirt bike at about 5.45pm on Thursday.

The rider was crossing a sporting field when he rode down a hill, hitting the girl and riding over her head and chest.

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The rider did not stop and rode away from the scene.

A witness rushed to the aid of the child, who had serious facial injuries.

Paramedics treated her at the park before she was taken to Westmead Children’s Hospital in a serious condition.

Police are appealing for witnesses to come forward, and anyone with information is urged to contact crime stoppers.

A crime scene was established and an investigation is underway, police said.