The two-vehicle crash, which happened at about 1.30pm on Monday, has forced the closure of State Highway 1 at the Selwyn River Bridge, south of Burnham.
St John spokesman Gerard Campbell said one person suffered critical injuries and was taken to hospital by helicopter. Two others suffered moderate injuries and have also been taken to hospital.
Fire and Emergency NZ spokesman Andrew Norris said crews attended and had to cut the person out of their vehicle.
Diversions were in place.