Auckland has had seven volcanic eruptions which have buried the area now occupied by the city under more than 10cm of ash, research has revealed.
"That amount of ash would cause chaos in Auckland," said GNS Science volcanologist Graham Leonard. "From a hazard point of view, that is pretty significant".
The research - which casts new light on volcanic threats to the area - springs from a drilling programme in the area.
A partnership between Auckland University and GNS Science, it was started to collect drill cores providing a record of long-term changes in climate, but the drill cores have also thrown new light on eruptions.
Some of the cores in the Orakei Basin have shown the Orakei volcano is probably three times as old as previously thought, and by drilling 81m into the sediment beneath the basin the scientists have identified ash layers from 90 eruptions in about the past 90,000 years.
Until now, scientists believed the Orakei volcano was about 20,000 years old.
The scientists have identified at least 50 different ash layers from Auckland volcanoes such as Little Rangitoto, Mt Hobson, the Domain, Mt St John, and One Tree Hill.