A New Zealand father was forced to choose between saving his wife or his son after the family car plunged into a river, he said on Monday.

Stacy Horton chose to go to his screaming wife Vanessa who was struggling in the water after escaping the sinking car, while his 13-year-old son Silva remained trapped in the vehicle about 4.5 metres below the surface.

"I tried to go down and get him out, I could not reach the car so my main priority was to get Vanessa to safety," Horton told commercial television.

"I got her to the edge of the river and on to the shore and looked back and could see the tail lights of the car and I just accepted he had gone, that there was nothing I could do for him."

The car had skidded down a grassy bank and over a 10-metre cliff into a river near the family home in Wanganui on the west coast of the North Island on Saturday evening.

Silva's friend Robert Palmer had escaped from the sinking car and raced to tell Horton what had happened.

'I could hear my wife screaming'

"I could hear my wife screaming in the water so I grabbed a torch from a man there and just jumped down."

After initially failing to reach the car, he decided he had to focus on saving his wife.

"I thought also if I did go down there and get in there... I may not make it and then my wife would not have made it either and it would have been my two girls without three of us."

After Horton rescued his wife, a police officer tried to dive to the submerged car but was unable to reach it.

"It was an incredibly traumatic incident for everyone involved," Sergeant Andrew McDonald of Wanganui police said.