Other transport operators had refused drive trampers to the track that morning due to the weather and police have said the shuttle company were "irresponsible''.
Mr D'Silva agreed the company were partly to blame but said his group were unprepared for the conditions.
"We've done a few bush walks and done tramping in India, but not as much as Kiwis do. I wouldn't call us outdoor kind of people. I think next time we will try something different,' he said.
"We started on thinking we would be able to handle a bit of rain, but it got progressively worse and once we were on top of the summit it was really bad; a girl collapsed with hypothermia, we didn't want to turn back because the winds were coming from that direction,'' he said.
Walking back to the start of the track would have meant walking into the rain.
"The visibility was really poor so we were scared we were going to be pushed off a cliff by the wind.''
He said the group didn't have the right gear - "half of us didn't even have jackets on, and they weren't waterproof,'' he said.
"We didn't wear what we should have for the trip, we underestimated the weather. By the time we realised we needed to go back, it was too late.''
Constable Aaron Owen, who coordinated the rescue, said the outcome would have been worse if the group hadn't come across two well-equipped trampers who helped them down the mountain.
"There were some in the group who were in pretty bad shape. They were incomprehensible, mumbling, and their eyes were rolling to the back of their head,'' he said.
Mr D'Silva said the group had been lucky to meet Ghaz Jabur on the track who is a clinical cardiopulmonary perfusionist.
"Ghaz took charge of the situation, he gave us food and he had a thermal blanket. We gave the collapsed girl a spare jacket and took off her wet clothes,'' he said.
"We hid behind a rock so that one of the girls could get warm,'' said Mr D'Silva.
The group of 16 walked down to the where they had been dropped off, and about 6kms from the start they met a team of five from Tongariro alpine search and rescue.
The rescuers had food, hot drinks, heat packs and dry clothing for the group.
"They took good care of us, calmed us down, split us into groups - walked us all the way - five search and rescue crew.''
Two women were taken to Taupo hospital with suspected hypothermia.
Mr Owen said the shuttle company was the only one in the area who wasn't a member of the Tongariro Alpine Crossing users group, which set protocols about when it was safe to tramp.
Mountain Shuttles owner-operator Kevin Gardiner, who was driving the shuttle, said he did not regret taking the group to the mountain.
"It wasn't too bad when we were up there in the morning, it was just the wind situation. But I actually read them the forecast which was showers clearing in the morning, westerly strong winds - 75km wind in exposed places - gradually dying away in the afternoon. So they were well aware of that.
"I saw no issue: we're talking about adults, people that make their own decision,'' he said.
Mr Gardiner questioned whether some in the group had been wearing shorts, as reported earlier: "I don't know about that, I don't know, that's a very sticky one. They were adequately dressed in my opinion, they even had winter hats on.''
He had also told the group that if they wanted to turn back at any stage he would pick them up.
Police reminded people venturing into the outdoors that the winter season is nearly upon us which means a dramatic drop in temperatures, especially in alpine environments.
