He noted that a year before Robert's death, a young woman was seriously injured while jumping from the falls.
The Wairua Falls reserve is administered by Department of Conservation. Whangarei conservation services manager Andrea Booth said there was a barrier at the top of the falls but staff would now assess whether signs were needed.
Ms Booth said as far as key staff were aware, the department was never formally notified about the drowning and the issue of signs had not been raised.
The coroner's report said that after Robert hit the water badly, Thomas and Matthew Pittam saw him about a metre under the surface, looking "lifeless". They plunged into the water but Robert was fast sinking beyond their ability to reach him.
During several attempts by the Pittams to locate their cousin under the water, other witnesses called emergency services.
The body was found the next day by a police dive team, at about 5.2 metres depth. A post mortem confirmed he had drowned.
Mr Shortland said that jumping into water off rocks, bridges and waterfalls are a part of summer life for many young New Zealanders.
Robert's was "truly a tragedy and a preventable death," he said.
Robert's mother Janet Fox said there were no surprises for the family in the report.
"We always knew it was that, just a terrible accident," Mrs Fox said.
Robert, from Donnelly's Crossing, 45 kilometres north of Dargaville, was familiar with the Wairua Falls and, having grown up on a farm, understood physical challenges.
"I am supportive of the coroner in that he said young people do those kinds of things, but about the signs, I don't think you can always make your kids safe by taking fun away," Mrs Fox said. "Yes, we lost a son but we wouldn't have wanted to stifle him."
