A survivor of the Cave Creek tragedy says he and several other students were shaking the Department of Conservation viewing platform hard when it collapsed.
Stacy Mitchell said he had carried an enormous burden of guilt and fear of being blamed for the disaster, in which 14 people died in April 1995.
He was one of 17 members of a party of students who crowded on to the platform high above Cave Creek, on the West Coast, with a DoC Punakaiki Field Centre manager.
The platform collapsed and fell 30m into the gully, killing 13 of the students from Tai Poutini Polytechnic in Greymouth and the DoC officer.
On last night's Paul Holmes show on Prime, Mr Mitchell said he walked on to the platform with his friends and looked down at the view.
The 18-year-old was one of several students shaking the platform, "just fooling round".
"It was shaking a lot. We were giving it a good nudge. But we felt safe doing it.
"There was a sudden movement forward and the platform just dropped. It tilted in a violent movement and we all fell forward against the handrail."
Mr Mitchell said he was knocked out on the way down and when he came to, someone was lying on top of him. "I was winded and in an enormous amount of pain."
He pushed the person off him. The person was convulsing.
Mr Mitchell said he felt like the instigator and the fear of being blamed kept him from talking to the police or to the commission of inquiry into the cause of the tragedy.
He said not a day had gone by without him seeing images of the scene and the people he was close to.
He believed that if the platform had been properly built it wouldn't have collapsed.
"A day before, a group had stood on the platform. It had been there for two years and there had been people visiting it and standing on it.
"I thought [afterwards] that if I hadn't shaken it maybe there was a chance that it wouldn't have fallen and killed so many that day."
Mr Mitchell said the hardest thing in dealing with the tragedy was not being able to tell anyone.
"I was living in denial. I didn't have a lot of people I could talk to because talking to them would mean telling them the truth. I was so scared of telling someone and then being blamed."
He said he turned to alcohol and cannabis in an attempt to escape his memories.
But about nine months ago he reached breaking point and sought help while living in the South Island with his two children.
The commission of inquiry into the cause of the April 1995 tragedy found that the platform was poorly designed and built by unqualified staff but that no individual DoC officers should be blamed.
The tragedy resulted from combined systemic failure against a background of underfunding.
Aucklander Graeme Hunt, who wrote a book about the disaster, said he had not heard any allegation that the platform fell due to students shaking the structure.
"It's a misplaced guilt - the platform could have fallen at any time.
"There was no question in my mind that it was unstable from the day it was built, whether someone had shaken it or not."