A witness to a crash involving a prison truck has described watching the horror unfold in his rear view mirror.

A prison truck with at least five prisoners inside has collided with another vehicle on State Highway 1 south of Kawakawa and one person in the other vehicle has died.

The man said he was driving directly in front of the truck when it collided with the car.

"I was watching in my mirror and saw it all happen behind me," he said.

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"It looked like the car was overtaking, and hit the truck head-on."

The witness, who asked to remain anonymous, described the impact as "major".

He said he knew the crash was serious when he saw glass flying, and the state of the car.

He had pulled over and called 111 before he continued on his way.

He wasn't sure if anyone had gotten out of the truck following the incident, as the truck was blocking his view.

Several other cars had pulled over too, he said.

One person who was travelling in the First Security escort vehicle is in hospital with moderate injuries, with a staff escort.

Four other prisoners who were in the truck have been taken to Northland Regions Corrections Facility.

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The crash scene on SH1. Photo / Supplied
The crash scene on SH1. Photo / Supplied

Andy Milne, Corrections Deputy National Commissioner, said staff from the prison responded following the incident, as well as off-duty staff.

"There was no risk to public safety," he said.

"All prisoners involved will be offered a call to their next of kin to let them know that they are safe."

Two First Security staffers were in the vehicle, and have also been taken to hospital.

Police and other emergency services were called to the crash, on SH1 near Callaghan Rd at Waiomio, south of Kawakawa about 2.45pm today.

A large number of emergency vehicles are at the scene. Photo / John Stone
A large number of emergency vehicles are at the scene. Photo / John Stone

It's understood the prison truck and a 4X4 vehicle collided, and the truck overturned, trapping people inside.

Police, Fire and Emergency and St John staff are at the scene.

The police cordon at the Ruapekapeka Rd diversion south of the crash scene. Photo / Northern Advocate
The police cordon at the Ruapekapeka Rd diversion south of the crash scene. Photo / Northern Advocate

A St John spokeswoman said four ambulances and one rescue helicopter were in attendance.

A Fire and Emergency New Zealand spokeswoman said they were called to the emergency at 2.45pm and two crews from Kawakawa were in attendance.

The highway is believed to be blocked while the victims are treated and the accident scene cleared. Diversions are in place through Ruapekapeka Rd, although police said it was not suitable for heavy vehicles.

The prison truck and another vehicle collided on SH1 south of Kawakawa. Photo / John Stone
The prison truck and another vehicle collided on SH1 south of Kawakawa. Photo / John Stone

Southbound traffic is being diverted at the intersection of SH1 and SH11 at Kawakawa.

Traffic is backing up on both sides of the accident scene.

Police said diversions would remain in place for some time and warned motorists to expect delays.