In his findings released today, Coroner Brandt Shortland recommended the local council consider putting up warning signs about the dangers of jumping from the falls.
Robert had gone to the falls with his girlfriend Nikita Hynes and cousins Thomas and Matthew Pittam, who leaped into the 7.4m deep pool below the falls several times that day.
On the fatal plunge, Robert made a running jump from a slightly higher point than usual. He fell awkwardly, feet-first, but with his head forward, which meant his head struck the surface of the water hard.
Thomas Pittam said Robert ran to the edge and took about two steps before launching himself off the ledge, which he believed unbalanced Robert and caused him to fall awkwardly.
He recalled his cousin's head snapping back as his face hit the water.
Thomas could see his cousin's body about a metre below the surface, looking lifeless. He and his brother Matthew jumped in to try to save Robert, but his body was descending too quickly.
Police divers later found his body slumped over a boulder about 5m below the surface.
Coroner Shortland said his death was truly a tragedy and preventable.
He said many young people in New Zealand jumped off bridges, waterfalls and rocks during the summer months, and Robert and his cousins and girlfriend had been enjoying themselves in that way.
Coroner Shortland found Robert's awkward and unbalanced fall meant his face and head had taken the full impact of his weight as he hit the water, which likely knocked him unconscious. He quickly consumed water and drowned.
Coroner Shortland said nobody was to blame and the death was purely accidental.
Andrea Booth, conservation services manager for the Department of Conservation in Whangarei, said an assessment of the area around the waterfall would take place to determine whether signs would help deter people from jumping from the falls.
"A barrier is in place [around the waterfall], so we need to weigh up the barrier versus signs,'' she said.
The barrier - which is regularly checked for safety issues - was in place at the time of Robert's death, she said, adding that people choose to climb over it.
"We are still really concerned about it and will do a re-assessment of the situation, a risk assessment balancing people's enjoyment of the outdoors with the natural risk that's present,'' she said.
