Two people died in one vehicle and three in the other when they collided on State Highway 3, near Ōhaupō, north of Te Awamutu, just before 4.45pm.
Inspector Jeff Penno told media that one of the cars involved crossed the center line.
“At this point we don't know why it happened. We know one vehicle crossed the center line… but we need to find out why that happened.”
It was a terrible event and emergency services' thoughts were with the family and friends of the victims who endured a long journey of pain and suffering, he said.
Police are not revealing who the victims are at this stage, but Panno said the victims' families have been informed.
Any impairment caused by drugs or alcohol was central to this accident, Panno said.
Initial indications were that in this particular case road conditions were not the cause, although the crash did occur in a 60mph speed zone where no center barriers were in place, he said.
Evidence suggests the safest survival speed for a serious crash on an undivided road is 50 mph, he said.
Center barriers saved lives, but they were difficult to retrofit and sometimes they just didn't fit, he said. Waka Kotahi's role was to determine priorities regarding the center barriers, although police did provide them with accident information, Panno said.
Police could not determine whether all of the crash victims were wearing seat belts, but some certainly were, Panno said.
Panno said police were doing their job to save lives.
“It is our job to ensure that people on the road obey the rules, are sober, alert and drive according to the circumstances and according to their capabilities.
“So when things go wrong, we naturally take it personally. But that also gives us the motivation and the passion to do what we do and if that driver chooses to exploit us for that, we don't react – we think back to this event and say that's why we do what we do.”
Waipā Mayor Susan O'Regan told RNZ's Morning Report that the site where the accident occurred was “not what I would say a notorious hotspot”.
The crash was “heartbreaking and gut-wrenching at the same time,” she said.
The crash site would have been horrific for those first on the scene and emergency services had done a great job responding to the “grim” conditions, she said.
She thanked the emergency workers for their service and those who happened to be first on the scene “for their quick action and the right thing to do”.