A member of the public has been hailed a hero for helping rescue a woman from a fiery crash on a Wellington motorway.

A witness who saw the rescue says a stranger risked his own safety to help the female motorist who suffered fatal injuries in yesterday's crash.

The Porirua man, who has asked not to be named, was driving north from Wellington on Sunday afternoon when he saw a car engulfed in flames on the side of the Centennial Highway.

He asked his friend to film while he slowed down, wondering if he could offer any help as a health professional.

"I witnessed a policeman and another person, and the guy was carrying a woman," he said.

"It was just horrific and tragic what I saw, and for at least half an hour I was just shocked."

The video footage shows the stranger and the officer carrying the woman to safety as flames engulf her vehicle, which had crashed into a pole. The Herald will not publish the footage.

He said the woman was carried from the burning car by a policeman and a member of the public, who he imagined would be traumatised by the experience.

"I think the guy who helped really needs to be appreciated for what he did, because it's not something a normal person can do.

"You have to be strong-minded or something like that, to carry a woman who is [badly] burnt.

"And also appreciate the police officer, what he did was really heroic and generous."

The single-car crash, located on the southbound lane under the Newlands bridge over-ramp, was reported to police at 4.45pm on Sunday.

Police confirmed on Monday morning the woman had died from her injuries, and an investigation into the circumstances of the crash was under way.

The man said he was so shaken by what he witnessed that he was unable to go to work on Sunday evening.

"I was shocked to see something like that ... I knew she was not going to survive because it was that bad."