Mr Johnston described repeated attempts to reach Dion's foot, saying that at one stage he was pushed over the face of the rock next to the waterfall by the strong current and into the pool.
"Dion put his hand out the side of the waterfall towards me ... I tried pulling Dion to the side of the waterfall to reduce the hydraulic pressure on him and hopefully free him. I was unable to move Dion even slightly.
"Dion was pulling on my hand which was using up his energy, and in turn, pulling his head out into the flow of the waterfall ... Knowing that we could not free him from this position, I reluctantly let go of his hand."
Dion was trapped for about three hours and amputation of his leg was considered.
He was eventually freed and airlifted to Dunedin Hospital but died the next day. Pathologist Dr Martha Nicholson said his death was due to immersion hypothermia.
Mr Johnston, who is in the Wanaka police search and rescue (SAR) squad and also a member of the Wanaka Land SAR swift water rescue team, responded to the emergency call at about 6.38pm.
Once he reached the gorge, it took him 10 minutes to reach the site.
"The scene that presented itself was one I met with disbelief. There was a 2-3m waterfall on the left of the narrow gorge and at the top of the waterfall was a foot sticking out approximately 50cm below the top of the rock on the right of the waterfall."
The waterfall flowed into a pool of turbulent white water.
He was told Dion was still alive and had been in this position in the water for about an hour.
Those at the scene had tried to free his leg but the water pressure was too strong.
The boy's right leg was facing upstream, wedged between two rocks by the water flow, and his upper body had gone over the waterfall backwards.
Mr Johnston read a statement from 15-year-old Ben Simmers, who was with Dion in the gorge that day.
Dion was his best mate, Ben said. They went up the river with three other teenagers in a group.
He was leading and Dion was at the tailend of the group. They stopped to check everyone was okay, discovered Dion was missing and found him trapped in the waterfall.
"My emotions were going everywhere ...one minute I was crying and the next I was praying," Ben said.
Ben described Dion as "one of those that go to the limit and know when to pull out."
Ben's father, Jeff Simmers, of Gore, was holidaying at Wanaka with his wife, three sons and six other boys, including Dion and his brother Cody, who were friends of his sons.
He was nearby when his son came back waving and yelling to come quickly.
"I wasn't envisaging anything like what I saw.
"There was a torrent of water going through the rocks where he was. He was under the torrent of water, pinned onto the rocks."
While Mr Simmers was trying to free Dion, the water pressure swept him over the 2m waterfall.
He climbed up again but the pressure of the water initially prevented him from reaching Dion.
"I don't know how long we were there ... all I knew was that his foot was moving and he was still alive."
The inquest was told that since the tragedy, a warning sign had been altered to emphasise the danger, saying "Swimming in the upper gorge is extremely dangerous".
But Mr Crerar said even this might not be enough, and suggested the Department of Conservation should place another sign closer to the waterfall.
"It's a very, very hazardous area and people need to be warned," he said.
The coroner adjourned the inquest and reserved his findings.
