Civil Defence bosses could be forced to beef up their evacuation plans for Auckland after alarming new evidence about the city's volcanoes.

Geologists have uncovered evidence of huge eruptions which each buried the region in ash.  They are warning that Aucklanders should be prepared for the same thing to happen in the next 50 years.

The testing is part of a project to improve understanding of the size and frequency of eruptions to help prepare for the next one.

"There is enough of a probability of ash within our lifetimes in Auckland for us to need to prepare for how we deal with ash in the city, and for the cleanup of that ash," vulcanologist Graham Leonard says.

Leonard has been taking core samples from the Orakei Basin that show unexpectedly heavy historic ashfalls.

Drilling has identified ash layers from 90 eruptions over the past 90,000 years.  Eight of those layers are more than 10 centimetres thick, which would create a significant hazard and cause chaos in Auckland.

His findings have got Auckland's Civil Defence leaders thinking.

"We may have to look at some of our plans, and we may have to ensure that the way in which we evacuate Auckland, in the case of a volcano, is done on a far greater scale than was previously thought," Auckland Civil Defence manager Jim Stephens says.

Drilling finished on Friday but the investigation of Auckland's volcanic future will continue.