A seventh person has died overnight after the horror crash in Waverley.

A young girl who died after the horror crash that claimed the lives of her baby sister and stepfather had just celebrated her 8th birthday.

Nivek Madams was the back-seat passenger in a car that crashed head-on with another, killing all of its occupants near Waverley Racecourse yesterday.

Hospital staff ensured Nivek's relatives could be by her side before she died.

Nivek's 5-month-old sister also died in the crash, which killed seven people, including all four occupants in the other car.

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Only the children's mother survived. She is in a stable condition in Wellington Hospital's intensive care unit this morning, a spokeswoman said.

A family member said a phone was put to Nivek's ear in her hospital bed so her father and other relatives could speak to her.

The eight-year-old was now with her baby sister in heaven and the family was grieving the loss of their two "precious princesses".

Hospital staff told Nivek's family there was nothing more they could do to save her, as they scrambled to be by her bedside.

The crash which also took the life of a newborn baby, happened on a crisp and sunny winter's day in South Taranaki when conditions on the road were good.

Emergency services were called to a horror two-car crash on State Highway 3, Waverley, just after 11am yesterday.

Police initially reported five people had been killed in the crash, but later confirmed that a sixth person - believed to be the newborn - had also died.

Four elderly people in one vehicle, heading northbound towards Hawera, died instantly.

A male driver of the second car with the newborn baby, heading southbound, died at the scene and a woman and an 8-year-old girl from the same vehicle, were airlifted to Wellington and Waikato hospitals, respectively.

The woman remains in a critical condition.

Yesterday morning, Taranaki police had issued warnings to drivers after four crashes caused by ice on roads. Police are still investigating the cause of this crash.

The Waverley crash is the worst crash on New Zealand roads in 13 years.

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 yesterday's nightmare crash.

At the scene yesterday afternoon two vehicles could be seen on the road still - one of them covered with large pieces of tarpaulin.

The other car's windows had blown out and the front of it was crumpled.

Central District road policing manager Inspector David White had one word for the accident: devastating.

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."

He said emergency crews from St John, Fire and Emergency and Police at the scene would be offered support at some point, given what they had seen.

"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.''

White also paid tribute to those who were first at the scene, including members of the public who did what they could to help and alerted authorities.

"I really want to thank those who were first at the scene for their prompt action - coming across what must've been an absolutely horrible scene to come across."

A local woman told the Whanganui Chronicle she had left work about 500m from the crash and described the scene as "a mess''.

She said it appeared a vehicle had crossed the centre line and collided head-on with the second car.

The woman said the road was wet and slippery.

A staff member at the Waverley Four Square also described the road as wet and slipper, after a big frost yesterday morning.

However, authorities later said conditions on the road were fine and dry.

The circumstances of the crash are now being investigated by the Police Serious Crash Unit.

Although officers were still working to determine various factors - including whether speed was one - what was clear was that the vehicles had collided head-on.

Authorities said earlier it appeared one of the vehicles had crossed the centre-line on a slight bend in the road.

Police said they were working to contact the victims' next of kin.

SH3 Waverley was closed for several hours after the crash and traffic was diverted. The road was reopened about 6pm.

Yesterday's crash came after a truck driver was killed in an accident near Waverley on Monday.