A woman was flown to Waikato Hospital in a critical condition today after a road sweeping truck collided with the driver's side of her vehicle on State Highway 25, near Thames.

The male truck driver narrowly avoided a power pole and ended up in a nearby field, while the 25-year-old female driver of the car was trapped in her vehicle for about 15 minutes.

Fire fighters from Thames and Turua cut her out of her Ford Laser after it was t-boned by the front of the truck.

The accident happened at 5.10pm outside the Kopuarahi Community Centre on SH25, about 5kms west of the Kopu Bridge.

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SH25 between Hauraki Road and Bush Road were closed and diversions were in place.

Auckland Westpac Rescue Helicopter attended the crash and witnesses told the pilot the driver had appeared to do a u-turn in front of the truck. "The truck driver, who appears to have been driving the truck along the straight, managed to narrowly avoid a power pole and ended up in the paddocks."

Thames Fire Brigade station officer Ken Brokenshire, who attended the accident, said it was a "T-bone" crash. "The front of the truck went into the driver's side of the car and crushed that side."

The two men in the truck were uninjured.