The horror crash that claimed the lives of six people today in South Taranaki is the worst crash on New Zealand roads in 13 years.
Five of the victims of the smash near Waverley are adults and one is a newborn baby.
Police earlier said five people were dead but just after 3pm they confirmed there were now six fatalities.
The worst fatal crash in New Zealand's history was recorded in 1963, when 15 people were killed in a bus crash in Northland.
In 1995, eight people died in a house bus crash in Hawke's Bay.
In 2005, nine people died in a collision between a tourist van and a truck in Matamata-Piako.
There have been 10 crashes involving six fatalities since 1972, including today's crash.
The crash happened on State Highway 3, Waverley, after 11am and the highway reopened just after 6pm. Authorities thanked members of the public for their patience and urged them to "drive safe'' as traffic started through again.
All four elderly people travelling northbound were pronounced dead at the scene.
The male driver of the second vehicle - heading southbound - and a newborn baby in that car also died at the scene, police said.
The front-seat passenger of the southbound vehicle, a woman, was airlifted to Wellington Hospital with critical injuries.
An 8-year-old girl, who was in the back seat, was taken to Waikato Hospital for treatment.
At the scene this afternoon, Central District policing manager Inspector David White said police were working to notify the victims' next of kin as well as working to determine the circumstances of the crash.
Acknowledging the fine day around him, he said: "That this happened on a day like today is just unbelievable and just devastating for all those involved.
"I really want to thank those who were first at the scene for their prompt action - coming across what must've been an absolute horrible scene."
He said although officers were yet to determine what had happened leading up to the crash, it was clear the vehicles had collided head-on on the slight bend in the road.
He thanked members of the emergency services - St John, Fire and Emergency and police - who were first to respond to the scene; acknowledging they too had had to see and deal with a difficult situation today.
"Our thoughts are with the families. But for our staff, we have feelings too. We will deal with those involved today, but later on we will provide for our staff who need it.
"Dealing with a crash like this today, with six people dead, is just devastating. There are no other words for it, really.
"A lot of our staff have young families and having to deal with that is not pleasant. But our thoughts are with their families first. They're the ones who have actually lost the baby."
Police said the serious crash unit is now investigating.
At the time of the crash, the weather was clear and the road was dry. The crash occurred on a slight right-hand bend of the road, police said.
"The tragedy will affect a large number of people in our community. Our sympathies and condolences go out to the family, friends and loved ones of those involved,'' a statement said.
Officers said the priority now was making sure support was provided to those who need it after the "heartbreaking'' crash.
Whanganui Sergeant Colin Wright said earlier that four people were in each car that collided head-on on State Highway 3.
The car carrying the four elderly people was travelling west, towards Hawera, when the two cars collided near Waverley Racecourse.
"At the moment we're just investigating exactly what's happened."
It was earlier reported two rescue helicopters landed at the crash and a third was on its way. Two fire engines, three ambulances and multiple police officers were at the crash.
Wright said the road will be closed for a number of hours.
Cars are being diverted off SH3 down Ihupuku Rd and motorists are advised to avoid the area.
A local woman leaving work 500m from the crash described the scene as "a mess".
She said it looked like a vehicle had crossed the centre line and collided head-on with another vehicle.
The road was wet and slippery, she said.
A staff member at Waverley Four Square said the roads were still slippery after a big frost this morning.
Waverley is in the South Taranaki district, 44km northwest of Whanganui.
Taranaki police this morning warned drivers about ice on the road, after four crashes since 5am.
"We need motorists across Taranaki to be extremely cautious on the roads this morning after four crashes caused by ice on the road since 5am," police said.
"Any patches of road which appear wet will likely be ice and should be treated as such.
"Police are also advising people to check their vehicles and tyres are roadworthy and ensure all windows are completely clear of ice before getting behind the wheel."
Today's crash follows a fatal truck smash in Waverley on Monday.
As of yesterday 187 people have died on New Zealand roads so far this year, up from 183 at the same point in 2017.
Over the past 12 months 382 people have died on New Zealand's roads, up from 343 in the previous 12 months.