Hopkins had been given the trip to Whakaari for his birthday by his daughter, who' is a geology student. He believes they might have been the only Kiwis on their trip.
After they'd visited the island itself, their boat moved around to get one last look at the crater.
"As we turned to start heading back, there was just this gasp across the boat and I looked up.
"I could just see this plume of white and grey rising quite high and quite quickly," Hopkins told the Herald.
And yet, it was silent.
"At that moment, it was quite beautiful - we were watching a volcano erupt in front of our very eyes.
"But then the ash just rolled up over the rock face and as it rolled over, it just suddenly became quite menacing."
As the ash fell and the blue sky returned, they could see people who'd run into the sea to escape the eruption.
The crew launched the inflatable and rushed over to pull the people out of the water. In the background, Hopkins could see the destroyed helicopter and said it looked like it had been spray-painted grey.
When the first boatload came back from the island, someone yelled: "Is anyone a doctor?"
There were two - one from England and another from Slovenia - while Hopkins, a pastor from Hamilton, and his daughter Lilliani are trained in first aid.
At some point, someone made the decision to head back to the mainland with the 23 burnt people who'd been rescued from the island. They all needed treatment.
Five of them were critical.
Hopkins spent a lot of his time with a young tourist couple who were drifting in and out of consciousness. He tried to get as much information about them as he could so he could tell paramedics.
"My fear now is that they didn't make it. There were five critical people on our boat and there's been five fatalities confirmed … they were just so badly burnt."
Halfway back to the mainland, a coast guard boat dropped off some paramedics who tried to give people pain relief.
But they struggled to find veins under their burned skin as they bounced over the waves.
"I have no idea how long it took to get back, we just motored as fast as we could. But at this stage people started to go into shock.
"And they're cold because they're wet from pouring water on their burns.
"So instead of asking for water, we were asking for coats and jackets. People were taking their T-shirts off to give us clothes to try and keep people warm on the way back.
"I remember three Asian tourists that huddled together under a big waterproof duffle bag."
When they got back to shore, the paramedics took over while a bus took the uninjured back to the White Island Tours offices were they were met by officials.
They were helped with getting in touch with loved ones.