Police say they know how a head-on two vehicle crash that left five people dead in Waikato on Tuesday afternoon happened but are still investigating why.

Two people died in one vehicle, and three in the other, when they collided on State Highway 3, at Ōhaupō, north of Te Awamutu just before 4.45pm.

Speaking to media, Inspector Jeff Penno says one of the cars involved crossed the centre line.

"At this stage we don't know why it occurred, we know one vehicle crossed the centre line ... but we need to work out why that occurred."

It's a horrendous event and the thoughts of emergency services are with the family and friends of the victims who are on a long journey of pain and suffering, he says.

Police are not releasing who the victims are at this stage, but Jeff says families of the victims have been informed.

Any impairment by drugs or alcohol is a focus in this accident, says Jeff.

The scene of Tuesday's fatal crash on State Highway 3, north of Te Awamutu. Photo: RNZ / Natalie Akoorie.

Early indications are that roading factors are not at fault in this particular case, although the crash did take place in a 100km/h speed zone where there are no centre barriers in place, he says.

Evidence suggests the safest survivable speed for a serious crash on an undivided road is 80km/h, he says.

Centre barriers do save lives, but they are hard to retrofit and sometimes they just did not fit, he says.

It's the role of Waka Kotahi to determine priorities regarding centre barriers, although police do feed them with crash information, says Jeff.

Police can't determine whether all of the crash victims had been wearing seatbelts, but some certainly were, he says.

Jeff syas police do their jobs in order to save lives.

"Our job is to make sure people on the road are compliant, they're sober, they're alert and they are driving to the conditions and to their ability.

"So when it goes wrong, yes we do take it personally. But equally that gives us the motivation and the passion to do what we do and if that motorist chooses to abuse us for that we do not respond - we think back to this event and say that is why we do what we do."

Waipā Mayor Susan O'Regan told Morning Report that the spot where the accident occurred is "not what I would say is a notorious hot spot".

The crash is "heartbreaking and gut-wrenching all at once", she says.

The crash site would have been horrendous for those first on the scene and emergency services have done an amazing job of responding to the "grim" circumstances, she says.

She thanks emergency workers for their service and those who just happened to be the first people on the scene "for acting promptly and doing the right thing".

-RNZ.