One person has died and multiple people are injured after an accident involving three cars that closed State Highway 1 south of Waiouru.

Police were alerted to the crash on SH1 in the Waiouru Military Area, in the Ruapehu district in the central North Island, about 11.45am today.

A St John spokeswoman said they were transporting seven patients: one in a critical condition, two in a serious condition, and four with moderate injuries.

A witness to the crash said he was driving north behind a car when it attempted to overtake a truck on a blind corner.

"It went head-on into a car coming the other direction," Alex Campbell told the Herald.

"I got out and went over. My adrenaline was pumping really hard and all I was thinking was 'how can I help the people in this situation and get this family out as quickly as possible'.

"I helped the driver out of his seat. I then proceeded to help his family out of the vehicle.

"There were two adults in the back, a mother and possibly grandmother.

"I had to pin the passenger front door open, bend the top of it and break the window to get an infant girl out of there."

Campbell said another child and baby were also removed from the car.

SH1 WAIOURU – CRASH – 12:30PM
Due to a #crash, #SH1 is CLOSED south of Waiouru. Due to roadworks, #SH4 is also CLOSED between Whanganui & Raetihi. Crews are currently working to re-open #SH4 but please note it may not be possible for safety reasons. Please DELAY your travel. ^EL pic.twitter.com/QpllQfS0n6

— NZTA Central Nth Is (@NZTACNI) June 13, 2019

He said there were at least two people in the other vehicle.

"The passenger was trapped inside, so we had to wait for about 20 minutes for the ambulance to come with the jaws of life."

Campbell said the front of both vehicles was caved in.

He said the truck appeared to have continued on and was not involved in the incident.

A rescue helicopter had been sent from Palmerston North.

The road was closed and motorists were advised to avoid the area.

The Serious Crash Unit had been advised.