Barrie Drummond, a former railway worker who lives 4km from the crash site, arrived there about 7.30am. He said the vehicle had ended up on railway tracks at the bottom of the steep bank.
"I could only see the top of the van from the road but it was squashed like a tin can.
"It couldn't have been a worse morning for fog and it was lucky that a coal train coming from Westport was stopped about 10km from Arthurs Pass or it would have been a terrible disaster," Drummond told the Herald on Sunday.
"If that train had kept going, it would have cleared out the van and all the other people who were injured. It would have been carnage.
"I was told the driver somehow scrambled up the bank through the bush and managed to stop a passing milk tanker for help," he said.
"I believe that the woman who died was found in the river that runs alongside the railway lines.
"Because of the fog, the helicopters couldn't winch people to safety and they were stretchered about 500m to a point where there was access to the road."
A crew member from the New Zealand Coal and Carbon Ltd rescue helicopter, which flew to the scene from Greymouth, said the vehicle was mangled beyond recognition.
"I couldn't really tell what sort of vehicle it was. We think it was [a van] but it's too hard to tell; it's taken a fair old bang."
Last night, a male and a female were in a critical condition in Christchurch Hospital's intensive care unit. Two males and a female were in a stable condition in the emergency department.
The other fatal crash on State Highway 73 occurred near Porters Pass in the foothills of the Southern Alps about 5.30pm. A woman driver was killed when her station wagon and a bus collided.
The bus driver was also hurt. St John Ambulance said his injuries were "minor to moderate".
Police and the Fire Service said neither the car nor the bus had passengers.
- additional reporting: John Weekes
