Specialist soldiers working to recover eight bodies on Whaakari/White Island on Friday were pushed past limits of normal exertion in battling the challenging conditions.
The team braved the elements on the island to retrieve six bodies. Authorities are still searching for the remaining two bodies, thought to be at sea.
Defence Force special operations component commander Rian McKinstry told TVNZ1's Breakfast today the team was pushed to the end of its tether in the "very tough conditions".
"Atmosphere was difficult," he said, talking about the need for breathing apparatuses and 15-kilogram protective gear.
"There was heat - heat that was generated just from heat of the day, heat from the protective equipment that the team were wearing and then the heat just generating form the volcano."
He also said rain mixed with ash on the island created a thick sludge - adding the the challenge soldiers faced navigating the area. They also had limited use of their hands and feet, reduced visibility and reduced communication in their suits.
When asked by Breakfast host Hayley Holt if they ever thought they couldn't complete the task, Mr McKinstry said yes.
About an hour into the operation, the decision to bring the team back to the jetty was made. They were looked over by a doctor and rehydrated, as well as put water on their bodies to cool down.
A plan was then worked out on how to spend the next two hours to ensure they got the task done.
"One of the things when the team went onto the island was there were still a few unknowns - and that was how they would perform. They were certainly trained on the equipment and trained to work in adverse conditions, but nothing quite like the conditions that we asked them to operate in on Friday," Mr McKinstry said.
"The team were anxious to do a good job. At the same time they were all very well prepared and very keen to be the ones to get the job done. In fact, we had a second team on standby who were perhaps disappointed that it wasn't going to be them.
"Then at the end of the operation - a huge sense of relief that they'd got the job done because they did have to go what I believe are past the limits of normal exertion to get the task done. So it was just relief and tiredness, physically just at the ends of their tether."
Throughout their military careers, the team had worked on building mental resiliency in difficult environments, Mr McKinstry said.
"They've also had the unfortunate task of having to serve in other tragic situations to, I guess, give them that experience that they needed on Friday."
They'd also trained for years with the specialist equipment, he said.
"We've reflected is on our performance on Friday - what we can learn about the equipment. We were able to pass on some very helpful information to the New Zealand police that allowed them to make some detailed plans around a revisit to island yesterday, and then of course for any continued efforts."