An Auckland eruption could force the evacuation of more than 400,000 people - but the biggest risk to life would come with a fast-rolling, bomb-like surge of hot rock and gas.
That's according to a researcher who has begun a new Earthquake Commission-funded study modelling what impacts a big blow in the city-wide Auckland Volcanic Field would have on homes and businesses.
"There's a huge amount of research on how volcanoes behave," Massey University's Dr Stuart Mead said.
"What we want to do is use that knowledge to get a more accurate understanding of the effect of an eruption in the Auckland volcanic field.
"The aim is to provide emergency managers with simple and clear information to support their decisions about how many people to evacuate if an eruption threatens."
The deadliest threat was the "surge" - a 200-600C mixture of fragmented rock and gas travelling along the ground at around 200km/h at the beginning of the eruption.
"The surge affects a smaller area than the ash fall, but inflicts a lot of damage. We compare it to a bomb blast," Mead said.
"It's vital to make sure that people are not in the surge area when an eruption happens. And we know from earlier research that most Auckland Volcanic Field eruptions have had a surge."
Volcanoes typically gave signals when about to erupt, such as a series of earthquakes that occurred as magma rose through the earth's crust.
In Auckland, current planning meant evacuating up to a 5km radius which could restrict up to 435,000 people, although this also allowed for a smaller or larger area to be evacuated if better information was available.
"When we look at research into the history of Auckland volcanoes, we see that some surges have been smaller than the 5km radius."
Further, as the surge flowed, it was affected by terrain such as hills, potentially changing the affected area.
"What we're doing is taking the research that's already been done on surges, and using computer modelling to get more accurate information about where a surge would travel and create damage," Mead said.
"We are expecting to be able to better define surge damage zones, which could assist in both long-term and evacuation plans.
"Given the potential impact on people, industry, road and air travel and tourism, this information could be very useful."
Once his team has crunched the data, they will create a model for emergency managers.
"We've got experts on our research team from computer science, statistics and volcanology," he said.
"What we are aiming to show in the model is the likely effects of a surge, and our confidence level in the prediction, so that emergency managers can weigh up the situation and make the best decision."
EQC science and education manager Dr Richard Smith said the project was a good example of using scientific research for local action.
"Dr Mead and his team will make a very practical contribution to volcano risk management in New Zealand," he said.
"Getting a better understanding of where and how far a volcanic surge is likely to travel will help Auckland's emergency managers be more effective in managing evacuation."