A driver died after two vehicles collided head-on this afternoon just west of the Kopu Bridge near Thames.
The other driver, a woman, remains in a critical condition.
Four people were injured - two badly - in a separate car crash when their car careened down a hill on the Kopu-Hikuai Rd, also near Thames this afternoon.
State Highway 25 between Kopu Bridge and the Turua turn-off and SH25A, west of Thames have reopened, but police are urging motorists to take care or postpone or use an alternative route via Paeroa for at least two hours.
Police understand that a silver station wagon travelling west from Thames apparently crossed the centre line and collided head-on with a green four wheel drive. The crash happened about 1.25pm on SH25 west of Kopu Bridge before the Turua turn-off.
The driver of the green car died at the scene and the driver of the silver car was flown by helicopter to Waikato Hospital and remains in a critical condition.
A Thames woman arrived at the crash just minutes after it happened and said she was talking to a local man who was travelling directly in front of the green car on his way to work in Thames who said the silver car only narrowly missed him.
"He was pretty shaken up when I spoke to him... He said that they'd nearly hit him..."
The woman said the people remained trapped in their cars for some time.
Fire crews from Thames and Ngatea cut the two trapped motorists from each of their cars, Northern Fire communications shift manager Dallas Ramsay said.
Paeroa woman Jill Barrass got caught up in the traffic on her way to Thames. She wondered why cars were "nose to tail". Then she saw the crash's cordon and understood.
"There was just a line of cars. I've never seen it like that before."
Barrass thought the traffic would have added at least another 30 minutes to people's trips. She said fellow motorists were considerate and slowed down once they realised it was a crash site.
"I was shocked that somebody died. It's still only just the beginning of the year. Those families are never going to be the same again. It's awful."
A resident who lived 200m from where the crash happened on SH25 said he didn't hear or see it, but that "traffic was banked up for miles".
About 15 minutes after the first crash, on the other side of Thames, a car carrying parents and a teenage son and daughter careened down a bank on Kopu-Hikuai Rd, between Kirikiri Valley and Puketui Rds. It was the only vehicle involved.
A St John spokeswoman said two rescue helicopters were at the scene and flew the mum and her 14-year-old son to Waikato Hospital's emergency department.
The boy received multiple serious injuries and was stabilised at the scene before being flown to Waikato hospital by the Trustpower TECT Rescue Helicopter for further treatment.
The Northland Emergency Services Trust helicopter transported a seriously injured woman to Waikato Hospital. The helicopter was called due to the nature of the injuries and the distance by road to the closest appropriate hospital.
A third person was taken by ambulance to Thames Hospital with moderate injuries, while the fourth suffered only minor injuries.
Due to a serious crash SH25A east of Kopu is CLOSED. A detour is in place, expect delays in area due to this & earlier crash on SH25. ^LC pic.twitter.com/osgU4b2xQQ— NZTA Waikato/BoP (@NZTAWaiBoP) January 10, 2017
The serious crash unit is investigating both accidents including what caused the green vehicle to cross the centre line.
Police also reminded motorists to take care on the roads and make sure they were well-rested, free of distractions and drove to the conditions.