St John medical director Dr Tony Smith was one of the doctors onboard an Auckland-based Westpac rescue helicopter, when they were dispatched to Whakatāne.
After fuelling up at Whakatāne Airport, they headed to the island.
"We arrived to a large plume of smoke and a thick cloud of sulphurous ash, almost forming like a fog," he said.
"There was a thick yellow stain running out to sea and it was clear from the extensive staining of sulphurous ash that there'd been a significant explosion."
There was a window of opportunity to safely land on the island, just on the beach near where the boats that visit the island offload their passengers, Dr Smith said.
He and three intensive care paramedics spent about half an hour on the ground, with the helicopter remaining immediately overhead, in case they needed to leave quickly.
"We were able to establish that unfortunately nobody remained alive and at that point we left and returned to the mainland, because we knew that there were large numbers of very badly injured patients."
On the island, Dr Smith said there was an incredibly strong smell of sulphur, even through the respirator masks they were wearing.
"Any exposed area of skin, for example our necks, our faces and our eyes were stinging pretty severely from contact with the sulphur.
"We're certainly not left with burns, but we left with very sensitive skin, very sensitive eyes and very sore throats as a result of being in contact with the sulphur."
Back in Whakatāne, the next task was to get the injured who had arrived at the airport or at the harbour to hospital.
A number of helicopters and small planes, from as far south as Christchurch and as far north as Whangārei, were brought in to take people with serious burn injuries to hospitals around the country with specialist units.
The injured were taken to Auckland City, Middlemore, Waikato, Tauranga, Hutt Valley and Christchurch hospitals.
Dr Smith said the scale of the injuries was like nothing he had seen before.
"This is certainly the first time in my experience, and I've been with St John now for over 20 years, that we've had to deal with something like this.
"Most of the patients had burns, some of them had limb fractures, presumably from either being thrown against an object or having an object like a rock thrown against them.
"But most of the patients had burns and the burns ranged from moderate burns right up to very severe and life threatening burns as a result of a combination of hot ash and hot liquid."
Dr Smith said even a moderate burn would involve hours and hours of reconstructive and lifesaving surgery and days of intensive care.
"The ongoing treatment for these patients in hospital is very intensive and for any hospital to have to deal with more than three or four of those patients at any one time is a really big ask and we've got a number of hospitals that are dealing with or three times that number of patients with severe burns and so this is a really big incident for the healthcare system in New Zealand."