A woman and her young baby were airlifted to Palmerston North Hospital, where she was yesterday in a stable condition.
The baby had been transferred to Starship Hospital in Auckland, hospital spokesman Dennis Geddes said.
Corporal Simpson, who serves in the 1st Royal New Zealand Infantry Regiment at Linton, said himself, his partner, and his two teenage sons were on Sunday travelling from Pahiatua to Masterton and had crested Mt Bruce when they saw a line of cars in the distance.
As he drove closer, he saw two damaged vehicles. He parked his vehicle and left his sons in the car with his wife "so they didn't have to witness anything", he said.
Mr Simpson, who has first aid training, had treated a motorcycle rider who crashed on the Pahiatua Track about six months ago, he said, and on Sunday was again ready to help.
"I went straight to the driver to see how he was and stayed with him. He told me his name was Eric."
The driver of the other damaged vehicle was trapped but conscious, Mr Simpson said, "because I could see his arms moving from where I was". He reassured Mr Bird paramedics were coming and started a conversation to help calm them.
"We talked about where he was heading and whether he knew what had happened," Mr Simpson said.
"I checked him over, and asked him if he could move his feet or feel any tingling or anything like that. I held his hand. He asked if there was any blood coming out of his mouth but there wasn't.
"He seemed all right to me but he did complain of chest injuries. It looked like the air bag could have given him a bit of a knock when it deployed.
"He knew how serious things were and was trying to look around. I told him not to worry and to just stay still. There were three elderly guys in that 4X4 and he seemed okay. I thought he was one of the better ones. Then the medics turned up.
"It's a shock to hear he died. Quite a shock. He seemed all right at the time."
Mr Simpson said the speed of some drivers on that stretch of SH2 "is horrendous" although he refused to speculate on the cause of the fatal crash, which was near a moderate bend.
"All I know was those vehicles were write-offs, just mangled wrecks, and two people are dead. We all have to watch our speed and our distance and don't cross the centreline on corners."