The group had been well-behaved and were looking forward to the festival.
"They weren't out of control or anything like that. They were just getting ready to leave when we left. We wished them happy New Year and we went out.
"The boy climbed the pole, which is about 10 metres high, and tried to jump into the pool and landed on the edge," he said.
"It's terrible. A terrible accident."
An ambulance spokesman said staff were alerted to the incident just after 8.30pm.
The man was in a critical condition when paramedics arrived at the home. They spent 40 minutes working on him before taking him to Gisborne Hospital, where he died shortly after arrival, the spokesman said.
The death has been been referred to the coroner.