Key Points:

Two badly injured people were trapped in their cars before being cut free after a head-on collision north of Levin late last night.

After being freed a man and a woman were both taken to Palmerston North Hospital.

The crash, between two cars, one northbound, the other heading south about 11.25pm, closed State Highway One, about 3km north of Levin, for several hours.

"Both drivers were trapped in their vehicles for over an hour before they were cut free by fire crews and transported to Palmerston North Hospital," Sergeant Jeff Lyver, of Levin police, said today.

The 28-year-old man driving the southbound car was in a serious condition with leg, head and possible internal injuries.

The 37-year-old woman driver of the northbound car was treated for a broken arm, a broken leg and a broken ankle.

Diversions along Koputaroa Road were put in place for about three hours.

The serious crash unit is investigating.