A Waikato teacher decided to "do something decent" after witnessing a fiery crash near Otorohanga on Monday.

Scotty Cranwell rushed towards a smoking car and managed to help pull three people from the vehicles following the crash at 2.50pm on State Highway 31.

Cranwell told Stuff he was in the car directly behind a campervan when he saw a black car cross the centreline and collide with the vehicle.

He recalled rushing to the car on fire and wrenching open the doors, amazed that they could even open.

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"My initial thought was actually to look for children. I was opening up all doors possible," he told Stuff.

He said the sound of screams and the smell of smoke made him think "this could be it".

Cranwell told Stuff a woman stumbled from the car saying "my luggage, my luggage".

"I said we don't have time for your luggage, it's your life."

An elderly man was slumped against the steering wheel in the campervan, and Cranwell and his workmate managed to pull him to safety.

"I was just stressing because I'm trying to find the seatbelt and just waiting for one of these cars to explode, basically," he told Stuff.

Shortly after, flames shot up into the air.

"I just started yelling 'get up everybody, it's going to blow'."

A vehicle caught fire and it spread to another vehicle before setting alight nearby vegetation.

Fire and Emergency NZ shift manager Paul Radden said six crews battled the blaze that had been about as big as a rugby field.

Three people were taken to Waikato Hospital, one person was seriously injured, while two others suffered moderate injuries.

Cranwell said it was miraculous that no one died in the crash.

"It was as clean as a head-on [crash] can be, I guess," he said.

Police are looking to speak with anyone who witnessed the manner of driving of the vehicles involved preceding the crash, or anyone who witnessed the crash itself.

Those vehicles are a grey 2019 Volkswagen Tiguan, and a grey-and-white 2018 Fiat Ducato campervan.

Anyone with information should get in touch with Constable Leesa Anderson or Senior Constable Mark Strongman at the Te Awamutu Police on 07 872 0100 or Crimestoppers anonymously on 0800 555 111.