Emergency services were notified of the single car crash just before 3:30am this morning near the intersection of Tane Rd and Linton Drain Rd, Tokomaru.
The vehicle hit a bridge and left the road, landing in the water.
A St John Spokesperson said the person was taken to Palmerston North hospital in a critical condition.
Contractors are on scene working to remove the vehicle from the water and inspect the bridge.
