An Auckland volcano would create a probable 6km ring of death as the "base surge" of superheated gases and ash exploded outward at ground level, a newly published study says.
And the results indicate the number of people evacuated in the 2008 volcano emergency simulation – named Ruaumoko – was "much smaller than the one suggested by a rational cost-benefit analysis".
GNS chief volcanologist Gill Jolly, one of the paper's authors, said almost everyone caught in a base surge dies, for example, in the 1987 eruption of Mt Pelee on the Carribean island of Martinique, 26,000 people caught in the base surge perished and only two survived.
By studying the effects of Auckland's numerous previous volcanoes and analogous volcanoes worldwide, the authors estimated that a new volcano would have a base surge with a 3km radius. Jolly, however, said "you can't rule out the possibility of it going 6 or 7km".
The volcano that created Lake Pupuke on Auckland's North Shore had a blast radius of 3km, Orakei basin 1.8km and Motukorea (Browns Island) 1km.
Smaller volcanoes around the world had base surges of 250m but the largest volcanoes had surges with a 5km radius.
A worrying "outlier" in the data is that Rangitoto, the most recent volcano, was the biggest by far and outside the conventional volcano field.
A volcanic eruption would probably be preceded by a series of earthquakes up to magnitude 4.5 to 5 as the magma made its way to the surface, Jolly said.
"The worst case scenario is hours [of notice], but probably it would be weeks. We would start to see the earthquakes get more frequent."
Jolly said a big issue was "we don't know where it's going to come up".
It had been thought that old volcanoes were safe from re-erupting but evidence was mounting that at least some, such as Rangitoto, had erupted at least twice.
Jolly said the probability of an explosion increased on whether the explosion was "wet" or "dry".
The base surge relies on the interaction between magma and water so, if the volcano vent occurred in or near the Waitemata or Manukau harbour areas, the probability of an explosion rose dramatically compared to a land-based "dry" eruption.
The Ruaumoko simulation was based on an eruption in the area around the sewage treatment ponds near Mangere. The civil defence exercise showed an evacuation area into central Auckland as far as about Mt Eden, to Penrose in the east and Lynfield in the west.