A woman has died and a 21-year-old man has been arrested following a head-on crash near Greta on Monday morning.

Emergency services were called to the New England Highway at Harpers Hill about 8.50am on 20 July following reports of a two-vehicle collision.

Police were told a sedan and a utility had crashed head-on, blocking traffic in both directions.

The female driver of the sedan was treated at the scene by NSW Ambulance paramedics but could not be revived.

She is yet to be formally identified.

Police allege the driver of the utility left the scene on foot following the crash.

A 21-year-old man was arrested nearby a short time later and taken to John Hunter Hospital for assessment and mandatory testing.

Officers from the Port Stephens-Hunter Police District established a crime scene, which will be forensically examined by the Crash Investigation Unit.

The New England Highway at Harpers Hill was expected to remain closed for several hours while emergency services and investigators remained at the scene.

Motorists were advised to check Live Traffic NSW for information about road closures and diversions.

Police are appealing for anyone who witnessed the crash, has dashcam footage or may have information that could assist the investigation to come forward.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000 or through the Crime Stoppers website.