State Highway 6 has reopened between Renwick and Havelock tonight after a collision involving four vehicles.

Police said the collision this afternoon involved a blue utility travelling towards Blenheim and three cars heading in the opposite direction.

Marlborough highway patrol manager Sergeant Barrie Greenall said the ute appeared to have crossed the centre line, where it collided with a white Toyota car, one of the three cars going in the opposite direction.

A man driving the white Toyota had been cut from his car and airlifted to Wellington Hospital by the Nelson Marlborough Rescue Helicopter from Nelson. He was the only person in the vehicle.

The man driving the ute was still trapped about 3.15pm and emergency services were working to free him.

The Westpac Rescue Helicopter from Wellington landed about 4.10pm on standby to fly him to Wellington Hospital, Mr Greenall said.

Both the men were in a critical condition, he said at the scene.

The people in the other two cars involved, a man in one car and a man and a woman in the other, were not injured in the crash.

Two ambulances, three fire crews from Renwick and Havelock and three police units were at the crash scene this afternoon, as well as the rescue helicopters.

Traffic had backed up for at least a kilometre at both ends of the road closure.

The road was reopened to one-lane traffic about 5.30pm after the second helicopter took off.

During the closure, traffic was diverted along Rapaura Rd, State Highway 1 and Queen Charlotte Dr.

Havelock police and the serious crash unit are investigating the cause of the crash.

- The Marlborough Express