Four people, including a preschooler and baby, have been discharged from hospital following a serious crash near Waiouru yesterday.

A child was killed and seven other people were injured in the two-car collision, which closed State Highway 1 in the Waiouru Military Area, about 11.45am.

A Whanganui District Health Board spokesman said five of the seven victims of the crash were admitted to Whanganui Hospital.

They were three adults and two children – all from the same vehicle, he said.

"Last night we discharged a 24-year-old male, a 23-year-old female and a 3-year-old girl – all from the same family.

"So, currently at the hospital we have a 22-year-old female with a broken arm, and her baby.

"The baby has officially been discharged but as her mum is still in hospital we continue to care for the infant."

Two other victims were also treated at Waikato Hospital. The Herald is awaiting a response on their condition.

Yesterday, a St John spokeswoman said one person was critically injured, two were in a serious condition, and four had moderate injuries.

Crash witness Alex Campbell said he was driving north behind a car when it tried to overtake a truck on a blind corner.

"It went head-on into a car coming the other direction," 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.

He said at least two people were 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 crash.

The crash means that as of yesterday, 19 children aged under 15 had died in crashes so far this year, according to Ministry of Transport data.

The number has already outstripped last year's total, in which 16 children died - with 10 of those deaths between January 1 and June 12.