Three people died in the other car, a blue Falcon. Lucan Ryder, his wife Patricia Matthews and their daughter Monique Ryder, 9, of Dannevirke, were killed in the crash. Ryder's sons, Rico aged 4 and his 10-year-old brother, were injured and taken to Waikato Hospital.
The boy was now in Waikato Hospital's intensive care unit waiting to be transferred to Starship and was in a serious condition, she said.
His 10-year-old brother was in a stable condition in a ward.
The family were believed to have been heading to Benneydale, 10 minutes' drive away.
Henrietta Turner, who was among the first to arrive after the crash, said she was trying to "block out" the horrific scene. "The kids were all screaming for a long time. They would have been scared."
Motorists who stopped were trying to comfort the children but Turner also saw the driver who carried on through the crash site. "I reckon they were scared, too frightened to stop."
Appalling weather conditions hampered search efforts, with rescue helicopters sent away as low cloud, fog and heavy rain obscured vision.
It was left to those on the ground - ambulance and fire crews from Benneydale, Mangakino and Te Kuiti - to do what they could for the survivors.
Even those in the emergency services say their job would have been much harder if not for those motorists who chose to stop.
Among the first of the emergency workers to arrive was Benneydale chief fire officer Murray Patterson, who said the scene was "horrific".
"The cars were just a couple of mangled wrecks. I think we are all going to need to undergo a bit of counselling after that."
Patterson praised the truck driver who noticed the Falcon with the two surviving children in it was on fire. Grabbing a fire extinguisher, he was able to direct the foam on the underside of the car.
"If it wasn't for him things could have been a lot worse."
He also praised a North American tourist studying to be a doctor. When emergency services arrived, she was already assessing the injured.
"These were just passersby that never expected to be put in this position but they did well," said Patterson. "They did fantastic work. The whole thing makes you drive home slower that's for sure. I hope drivers read about it and take it to heart."
Te Kuiti chief fire officer Grahame Howell arrived to find people still trapped in both cars. His team had to cut out the side of the Diamante and push a board under the male driver to slide him out.
"He was conscious. He was talking to us but was in pain. He seemed quite aware." He said the man was "very calm" and appeared to understand what had happened but faced a "huge grieving process".
"It makes you think of your family afterwards. You don't think about it at the time - it's just about getting the job done and getting help for these people as quickly as possible."
Mangakino chief fire officer John Kelso said the Falcon had been squashed until it was as wide as "the length of your arm".
"It was very nasty. The guys are pretty upset. I think I am going to have to get in a counsellor for them tomorrow."
Waitomo District Mayor Mark Ammon said the stretch of the highway east of Benneydale was one of the most dangerous in the Waikato. There had been several deaths near the township in recent years.
"It hasn't got many passing opportunities, so you get stuck behind trucks and people take risks."
Ammon said road conditions would have been poor yesterday. "Up there it would be almost snowing. It's a pretty bad day for driving."
Councillor Brian Hanna said some of the road's dangerous one-lane bridges had been widened recently.
"The road has had several improvements but it is a road that is getting more and more traffic use, and a lot of logging trucks use it."
Benneydale shopkeeper Ame Khan said it had been raining all day, and the stretch of road was treacherous when wet.
"There are heaps of crashes, most of them are really bad ones as well. This time of year, the road becomes very slippery."
View Crash on State Highway 30 in a larger map
