A motorist has described a life-saving decision to switch lanes as a car drove towards her the wrong way on the Waikato Expressway - the same vehicle that moments later was involved in a deadly head-on collision.

The woman was travelling along the Waikato Expressway on her way to work in Auckland around 4am on Monday when she noticed a car driving towards her from the Rangiriri off-ramp.

 “I was very confused and I decided to change my lane promptly to the outer slower lane as I could see a car at a high speed coming towards me, I thought it was [their] side of the highway but [I] quickly learned [that] the vehicle was heading in the wrong direction of the highway towards me,” she said.

Traffic was detoured off State Highway 1 Waikato Expressway after a head-on crash involving a car driving the wrong way killed two people. Photo / Mike Scott

“To be honest if I did not change lanes before coming around that bend I believe I would of been the one in the head-on collision.”

She said the only other vehicle on the road at the time was a truck about 200 metres behind her.

The woman said she hoped the driver of the car would have turned around after realising they were on the wrong side of the road.

Emergency services at the scene at the fatal head-on collision on the Waikato Expressway. Photo / Mike Scott

It wasn’t until later in the day, when she saw the news, that she learned things ended differently.

“That’s when I broke down in tears knowing I [had] passed side by side next to this person before both drivers of their vehicles died.”

At this moment she also learnt that she had passed the other vehicle involved in the crash just a kilometre before seeing the car driving the wrong way.

“I’m heartbroken for the ones [that] died and their families, and I’m also grateful to be alive,” she said.

Emergency services responded to reports of the collision on Monday morning just after 4am.

Police confirmed drivers of both of the cars died at the scene.

Traffic being detoured off State Highway 1 Waikato Expressway at Rangariri after the crash. Photo / Mike Scott

The road was closed for most of the day which left motorists waiting in long queues as they went through the diversion.

A police spokeswoman said today that the circumstances of the crash were still being investigated and that this could take months.

They said often coronial inquiries will follow.