Mt Taranaki is "overdue" to erupt and researchers say when it does it is likely to cover much of the North Island in a blanket of ash and disrupt airports, power and water supplies.
The volcano has shown little or no sign of activity for 200 years, but new research by Dr Shane Cronin, of the Institute of Natural Resources at Massey, suggests it has erupted at least once every 90 years on average for the past 9000 years, with a major eruption every 500 years.
The research indicated the last major eruption was in 1655 with smaller eruptions recorded in 1755 and possibly the early 1800s.
Dr Cronin, who completed the research with PhD student Michael Turner, extracted and analysed core sediment samples from Lake Umutekai, about 25km northeast of the volcano.
The mountain's smaller eruptions were similar to that of Mt Ruapehu in 1995-96 and would have showered New Plymouth with pumice and rock fragments the size of raisins.
"An eruption of this scale would undoubtedly cause substantial disruption to much of the North Island, cutting power supplies, damaging transmission lines, water supplies and stormwater," said Dr Cronin.
North Island airline flight paths would be disrupted and the prevailing southwesterly wind was likely to move the ash cloud over Auckland, closing the international airport.
The ash could damage pasture, crops and orchards. It was likely to block air filters on milking shed cooling plants and limit farmers' ability to store milk - 20 per cent of the country's dairy cattle are in the Taranaki region.
Dr Cronin said there was only statistical evidence to suggest an eruption but "with volcanic activity, the past is the key to the future".
Regular monitoring should give at least six days and possibly up to a month's warning of an eruption.
Gary Bedford, director of environment quality for the Taranaki Regional Council, welcomed the research but disputed claims that an eruption was overdue.
The greatest risk to the area in the next five to 10 years was a pandemic such as avian flu, he said.
Mt Taranaki was taken "very seriously" and the council recognised it was a question of when, not if, an eruption happened, Mr Bedford said.
The council was part of GeoNet, which monitored the mountain using six seismographs in the region.
"We are assured by the experts the nature of an eruption from Taranaki would be a slow build-up and much more likely to be a steady release than a sudden violent eruption that devastates the world overnight, so that gives us quite a degree of reassurance."
Shane Bayley, emergency management officer for the council, said it had a volcanic strategy in place, which included six levels of alert.
Currently the alert level was at zero but if the evidence suggested an eruption was likely - close to level 3 - between 20,000 and 40,000 people would be evacuated from the area.
* At 2518m, Mt Taranaki is the second-highest volcano in New Zealand (Mt Ruapehu is 2797m).
* Taranaki has a major eruption every 500 years.
* The last big blowout was in 1655.
* According to Maori legend, Taranaki was banished from the Ruapehu, Tongariro and Ngauruhoe cluster in the central North Island after a fight for the affections of the beautiful Mt Pihanga. Taranaki fled to the coast.