A driver is on the run after a high-speed crash in NSW left one woman dead and another without her unborn baby.

The driver was behind the wheel of a Holden sedan when it collided head-on with a Hyundai sedan on Newcastle Rd at North Lambton, Newcastle, just after 2.40am on Sunday.

Police said officers patrolling the area had earlier spotted the Holden allegedly driving dangerously, and signalled for it to stop, but it failed to do so.

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A pursuit was not initiated due to safety concerns, and officers lost sight of the vehicle when it allegedly crossed onto the wrong side of the road.

Moments later, it collided with the Hyundai in the eastbound lanes of Newcastle Rd.

A 55-year-old woman, who was a passenger in the Hyundai, died at the scene.

She is yet to be formally identified.

The Hyundai driver, a 28-year-old woman who was 25 weeks pregnant, was treated by paramedics before being rushed to John Hunter Hospital in a serious condition.

She later lost her unborn child.

The driver of the Holden and a 17-year-old front passenger fled the scene before police arrived, leaving behind two teenage girls — aged 15 and 17 — who were in the back seat.

Both teens were treated by paramedics and taken to John Hunter Hospital under police guard, where they remained in serious conditions.

Police are now appealing to speak with a man seen in the area at the time.

He is described as being of Aboriginal/Torres Strait Islander appearance, aged in his late teens, about 165cm tall, of thin build, with brown hair and brown eyes.

They also wish to speak with the front-seat passenger who fled the scene.

Newcastle Road was closed eastbound at Arthur St following the crash, with drivers urged to avoid the area.

A crime scene has been established, and a critical incident investigation is underway.

Anyone with information, CCTV or dashcam footage is urged to contact police.