Police said the man was driving with his mother in the central North Island on Tuesday when their vehicle skidded off the road and down a steep bank.
"There was torrential rain. But what really set it off was the fact he was eating a chocolate bar and he started to choke," Hawke's Bayroad policing spokesman Grant Marshall told Stuff.
"He applied the brakes to pull over, but he got into a skid and that's when they went off the side of the road."
The pair suffered only minor injuries and hypothermia in the accident.
"We're gob smacked as to how they survived this," Marshall said.
The utility vehicle was a write-off and remained in the Waipunga River.