The Toll helicopter landed on an oval near the jail to transport the man to specialist treatment.

A spokeswoman for NSW Corrective Services said the man had been flown to Westmead Hospital.

She said the inmate, who was housed in B Wing of the jail, attended the officers' station around 1pm with facial injuries.

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He was taken to the jail clinic for assessment but his condition rapidly deteriorated and ambulance officers were called.

The man was treated at the scene by paramedics and transported by road ambulance to a nearby oval where the Toll helicopter was on standby to fly the man to Sydney.

Detectives as well as forensic services officers were called to the jail, with police investigations now under way.