Walker said the man had driven out of his driveway, straight across a gravel road near the marae and directly down a bank landing in the Manaia River. The river was full because it was high tide.
The car had not flipped and was upright when it stopped.
Emergency services were called to the job at 10.12am and the man was taken to Thames Hospital by ambulance in a serious condition.
Fire crew helped move him from a paddock where residents had dragged him to.
A police media spokesperson said a person called 111 at 10.12am saying the car was sinking.