The wreckages of a car and ambulance involved in a fatal head on collision show the impact of the early-morning crash.

Photos from the scene on Tīrau Rd (State Highway 1), near Cambridge, show the buckled front of each vehicle, and the ambulance's shattered windscreen and buckled wheels.

Police were called about 3.40am on Wednesday, and the road remains closed between Cambridge and Karāpiro.

An initial statement from police said a vehicle had crashed into the back of an ambulance but Highway Patrol Sergeant Matthew Crossan said at the scene that the two vehicles collided head-on.

The ambulance had been heading south and the car heading north.

The driver of the car died at the scene, a police statement said.

The critically injured driver of the ambulance was airlifted to Waikato Hospital. Another ambulance staff member, who was inside the vehicle at the time, was moderately injured.

There were no patients in the ambulance.

At 7.50am Te Whatu Ora, Health New Zealand Waikato spokesperson Andrew McAlley said one patient was in theatre in a serious condition and another patient was in a stable condition in the Emergency Department.

The Serious Crash Unit is investigating the scene and the road is expected to be closed for some time.

St John said the organisation is assisting police but its injured staff are the priority.

'Our focus right now is to support our people and our thoughts are with them, their wider family, and friends at this difficult time,” deputy chief executive of ambulance operations Dan Ohs said.

St John spokesperson Amy Milne said two ambulances, two rapid response vehicles, one clinical manager and one helicopter had responded to the crash.

One patient was airlifted to Waikato Hospital in a critical condition and another was taken to hospital in a moderate condition by ambulance.

Waipā mayor Jim Mylchreest said he hadn't heard much about the crash.

”It is an absolutely shocking accident and my heart goes out to everyone involved.”

Mylchreest wasn't completely sure where on SH1 it occurred, but said there was a history of serious crashes at the intersections along that stretch, by the Mobil service station and Piarere intersection, after the end of the Waikato Expressway.

State Highway 1 is closed between Cambridge and Karāpiro, Waka Kotahi said.

The detour is via Maungatautari Road and through Cambridge. Heavy vehicles are advised to use SH29/SH27.

-Bill Hickman and Jo Lines-MacKenzie/Stuff.