Five of the victims of the smash near Waverley are adults and one a newborn baby.
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, shortly 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 in one northbound vehicle 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.
The serious crash unit is investigating.
