During the inquest, the White Island Tours groups were referred to as Group 1 and Group 2, while the Volcanic Air Safaris group was known as Group 3.

Cronin calculated the time each group had to react to the eruption based on their distance from the vent and the speed of the "pyroclastic density current".

"Group 2, the closest group to the vent, first see the event at two seconds after [2.12pm]. The call to run is [six seconds later], and based on my calculation of when the current arrives to them it gives them 27 seconds to respond," he explained.

"They had very little time to make a decision, so they have to act on instinct. They have very little time to put training into practice. Outrunning is impossible. There is no chance to outrun this."

Even at full speed, Cronin said they wouldn't have made it far.

"I was trying to think about how fast someone could conceivably run in 30 seconds. Across the terrain we're experiencing there are loose boulders, rocks, hazardous fumaroles, there are streams. This is not a running track," he said.

"In 30 seconds, the average person can manage potentially around 100 metres on this terrain."

The majority of deaths on Whakaari were in Group 2; only three people survived their injuries.

Though only a short distance away, Group 1 faced much better odds and 18 people survived.

Cronin said the key difference between Group 1 and Group 2 was their access to shelter.

"Group 1 was slightly further away and they had the ability to get behind shelter.

"Unfortunately the situation for Group 2 was they were in an area where there was very little shelter," he said.

"They would've needed to run a lot further in a shorter space of time to get to reasonable shelter."

He said Group 2 only had access to small mounds for cover.

Group 3 was closer to the shore and four of the five members survived the ordeal.

Three of the survivors from Group 3 took shelter underwater, which Cronin said minimised their injuries.

"A factor that seems in hindsight to have been important was getting into the ocean. Three people got into the ocean and they suffered either no burns or very small burns," he said.

"The decision to head to the ocean and get into the water was, in this case, well advised."

Cronin said the tourists' clothing also played a significant role in their survival.

"Any way in which the skin can be covered provides a temporary thermal barrier," he said.

"Covering the skin delays the thermal effect, it also stops the chemical impact if the clothing material is resistant enough."

Those who removed the ash from their bodies quickly also had fewer injuries, he said.

"Group 2 survivors report being buried in tens of centimetres of ash," he said.

"Even if it's only a few centimetres, that ash is still hot, [and] it's still full of those acidic compounds, so getting up and shaking off that hot ash was very important."

He said some particles in the cloud were over 400C, and the average temperature was 200C.