Yasmine Lees watched helplessly from a car as a packed tour bus careered downhill, picking up speed and skidding round sharp corners, before rolling and coming to a halt just a metre from a cliff edge.
The 18-year-old was travelling behind the 40-seater bus carrying Asian tourists, which collided with another vehicle in Otira, near Arthur's Pass, on New Year's Eve.
A total of 36 people were in the two vehicles, and many were rushed to hospital, including two with life threatening injuries.
When Ms Lees and her friend began following the bus downhill, from Arthur's Pass towards the west coast, it was going about 20km/h, she guessed. But the young cafe worker from Temuka said it suddenly picked up speed and began taking corners with a recommended limit of between 25 and 35km/h at 50 or 60km/h.
"It started taking the corners very quick, to the point where the driver wasn't able to control it. It was leaving skid marks on the road, and it got to the point where it was doing very sharp turns just to try and get round the corners. It was tipping, and hitting the inner and outer barriers on both sides of the road," she said.
Finally, a barrier blew out from the impact of the bus, which skidded to a halt on its side, 1.5m from the cliff edge.
"I heard this loud bang and [saw] the bus tip and almost slide off the edge." Then Ms Lees saw that a second car was involved in the crash.
Adrenaline and instincts took over and she ran to the vehicle to help its passengers out.
"We saw a woman in the passenger seat screaming. We rushed to get her out. She was covered in blood."
The woman's father - the driver - was trapped behind the wheel.
"Half his face was torn off."
A third passenger - the woman's mother - was in the back seat with her eyes closed.
"I thought she was gone. She was obviously unconscious. I started speaking to her, saying 'can you hear me, you're going to be okay' and she opened her eyes. Once I saw that, it took a weight off my shoulders."
Ms Lees then looked around and saw the "total carnage" of the scene: a woman trapped under the bus, blood and glass everywhere, injured passengers who were screaming and crying, and traffic building up along the busy road on either side of the crash.
As traffic began to build up she ran back up the hill to warn motorists that there had been a crash, and they needed to slow down.
No cellphone coverage in the area meant that when a person offered to drive back to Arthur's Pass to call emergency services, Ms Lees accepted gratefully.
With little more than a basic first aid certificate, she said she felt helpless: "There were people whose injuries I couldn't help with, who were in severe pain.
"Everyone was covered in blood, down their backs and their faces. It reminded me of Frankenstein. There were some hands... just kind of hanging off arms.
"It was hard because I could smell the rubber. The smell just wouldn't leave. Even after we left and carried on to the West Coast, I still felt like I could smell it. I didn't want to eat, I couldn't sleep that night. It just shocked me.
"It was crazy. I've never seen something so brutal. It was scary for me. I have never witnessed a huge tragedy, I wanted to help, but it was so hard.
"I've never been in that situation - it did hit me back home. I've had a car accident in the past, and seeing that bus lose traction, it really hit me, and I knew it wasn't going to end okay. I broke down, I started crying, I was scared."
Though she said she was still traumatised by the incident, Ms Lees was amazed with the way many of the people at the scene acted.
"It was amazing, the amount of people there, coming together and wanting to help. We worked like a big family. We had no clue who anyone was, but we worked together so well." But it's the sound of terrified people screaming that still keeps her up at night.
"That's something I won't forget."