"I just told them straight. It's one of the things that a headmaster dreads. I just explained the facts and said what a tragedy it was and how they need to support each other," Mr Moses said.
"He was with some mates out at Makara. They were just having a weekend out there. He was climbing a tree and quite simply the branch went and he fell and never recovered.
"This was on Sunday morning about 8 o'clock."
Ms Moses said he thought Rohan died later that day.
"It's just one of those utterly tragic stories."
The school sent an email out last night, ahead of today's assembly.
"We want to make sure that there were no rumours. Sometimes when a kid dies there's thoughts they must have died of their own hand. This wasn't the case at all [with Rohan].
All we wanted to do is to make sure there were no rumours."
Mr Moses said pupils had been offered a room where they could go and receive help or counselling if needed.
He had spoken to Rohan's family and said they were "devastated".
"Our hearts really go out to a lovely family."
At this stage no school-wide memorial was planned, but there would be a Wellington College presence at the funeral, which was likely to happen on Saturday.
The death has been referred to the coroner.
