A sergeant in a marked car was patrolling Middleton Street for a stolen vehicle believed to be in the area about 2.20am on Friday, January 5.

"At some stage that police vehicle has collided with a pedestrian on the roadway, a 39-year-old woman," Northern Region Commander Assistant Commissioner Peter McKenna said.

Police believe at this stage the woman was not on a pedestrian crossing.

The 39-year-old was rushed to Kempsey hospital before being transferred to Port Macquarie then airlifted to John Hunter Hospital in Newcastle.

"It's believed she is in a serious but stable condition with injuries to her leg, pelvis and spine," Assistant Commissioner McKenna said on Friday.

Police said it was understood her injuries were not life-threatening.

The officer was alone in the car, and it's believed he was not under lights and siren, when the pedestrian was hit.

Early indications reveal the woman was not linked to the stolen car investigation, and was simply a bystander.

The Crash Investigation Unit was tasked to the scene at South Kempsey on Friday, along with local police, and combed the area for clues about how the collision unfolded.

A critical incident investigation has been launched, which will examine the circumstances surrounding the incident, and be subject to an independent review.

"The circumstances of the collision and how it occurred will form part of that critical incident investigation," Assistant Commissioner McKenna said.

The sergeant driving the police sedan was breath-tested, and returned a negative result.

Anyone with information, or with relevant dashcam footage, has been urged to contact Kempsey police or Crime Stoppers on 1800 333 000.