A witness said the driver of the red car was a woman, whose legs were trapped beneath the steering wheel.

A head-on crash on the Kapiti Coast closed State Highway 1 and left one person with critical injuries.

The road was reopened shortly before 2pm after the crash at Paekakariki, between the rail overbridge and the intersection with Paekakariki Hill Road, about 10.30am.

Fire crews at the scene removed one person, thought to be the female driver, from the northbound car.

SUPPLIED The crash at Paekakariki has closed SH1 in both directions.

She was flown to Wellington Hospital in a critical condition.

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A witness said the driver's legs were trapped beneath the steering wheel. She appeared to be unconscious as fire crews removed the roof of her car.

The witness was told the car had been swerving before the accident, and that this may have been due to the woman suffering a medical problem.

Four others suffered moderate injuries, Wellington Free Ambulance said. Two of the injured were being taken to Wellington Hospital, and the other two were treated at the scene.

If you are planning on travelling via Paekakari today please avoid or delay travel if possible due to a crash. https://t.co/vVS6xYbAcx — NZTA Wellington (@NZTAWgtn) October 22, 2016

CRASH: A crash on #SH1 Coast Road by Fisherman's Table is blocking lanes. Traffic is being diverted by emergency services. Expect delays ^MF pic.twitter.com/ohED52HxMF — NZTA Wellington (@NZTAWgtn) October 22, 2016

The driver of the southbound car was taken to Wellington Hospital by ambulance, with minor injuries.

Diversions were in place for northbound traffic through Ames St, and southbound over Paekakariki Hill Road.

Drivers are being warned to continue to expect delays while congestion clears.

Southbound trucks were being prevented from heading over the hill road, and had to queue at the scene for the crash to be cleared.