A man had to make the terrible choice of rescuing his wife or teenage son when their car plunged into a river at the weekend.

Stacey Horton saved his wife, Vanessa, and their 13-year-old son Silva drowned in the Whanganui River.

Mid-Central police communications manager Kim Perks said today it was a very tough call for Mr Horton. "I would certainly not have wanted to be in his shoes."

Mrs Horton, 35, was driving Silva, his best friend, Robert Palmer, 14, and the family dog when her Mazda MPV stationwagon went off Somme Parade in Wanganui and plunged down a steep 10-metre bank into the water on Saturday.

Mr Horton said he arrived at the crash scene, less than two minutes after the accident happened to hear his wife screaming in the darkness and to see Robert and the dog scrambling up the bank.

He tried to dive down to their vehicle, which was nose down, more than a metre beneath the surface, he told The Dominion Post.

"I tried to get down and get him but I couldn't, it was just too deep. And Vanessa was going under," Mr Horton told the newspaper.

"I made a call to pull my wife to safety. I looked back and I could see the tail-lights but it was too far and I couldn't get him.

"I just had to accept the fact that he had gone.

"Instead of going down and risking my life as well as my wife and son's, I chose to take V back and sat on the shore praying. It was all I could do."

Police and fire officers were also unsuccessful in rescuing the boy.

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

"Police officers in uniform were diving in to try to reach the boy and fire officers were doing the same trying to hook a rope around the vehicle to pull it within easier reach.

"At the same we had staff trying to comfort his distraught family on the river bank."

Two local divers eventually reached the vehicle, which had sunk in about 4.5 metres of water, but it was too late to save Silva, Mr McDonald said.

The boy's mother and friend were treated for shock and minor injuries and had since been discharged from hospital.

Wanganui senior station officer Gary Wilson said the water was dark and murky and the firefighters struggled to reach the car.

"We tried everything but to no avail. It was a longshot but it was worth the risk to try and save him."

Divers recovered Silva's body early yesterday. His funeral will be held in Taihape.