It could take less than 50 hours to safely evacuate several hundred thousand people in the event of an impending Auckland eruption - provided the threat was well understood.
That's according to a new modelling study that explored how long it might take for people to move out of harm's way before a hypothetical big blow in the city.
Auckland straddles a large volcanic field home to more than 50 volcanic centres, posing an ever-present hazard to the city's 1.6m-strong population.
While its existing volcanoes are thought unlikely to erupt again, the Auckland Volcanic Field is young and potentially active - with the potential to bring fast-moving surges of hot rock and gas, and widespread ashfall.
Recent research has suggested signs of an imminent eruption could occur with anywhere between five and 15 days' warning, forcing the evacuation of perhaps more than 400,000 people from their homes.
But the new study, led by University of Auckland PhD candidate Alec Wild, focused on evacuation timelines well inside that window.
To address the unpredictability of any future eruptions, Wild and his co-authors created a model using demographic information, like population distribution and vehicle ownership, as well as geospatial tools.
That helped them understand how many people might be at immediate risk during a volcanic eruption, and how long it would take for them to evacuate away from danger.
They found Auckland's residential suburban areas would require the largest and longest evacuations.
Up to 320,000 residents may need to evacuate when the location of the new eruption is fairly well understood, which could take up to 49 hours to complete.
However, when the location of the impacted area is less certain, both these numbers increased.
The simulated evacuation considered four phases: the time taken to decide to call an evacuation; the public notification time; the evacuee's time to prepare; and the evacuee's travel time to beyond the evacuation zone.
The modelling found the length of time involved in the first three phases were all independent of wherever the vent emerged - and that all could be likely completed within just 36 hours.
Using transport data, the modelling found travel times to beyond the evacuation zone ranged from less than an hour to 11 hours, depending on traffic congestion, and when there was no uncertainty about the state of the vent.
"By combining the times modelled for all four phases, we found that when there is high certainty in the vent location, the median total evacuation clearance time with no congestion is approximately 37 hours," Wild and his co-authors reported in the Journal of Volcanology and Geothermal Research.
But when there was more uncertainty about the vent, the clearance time could grow to between 38 and 55 hours, again depending on traffic congestion.
A vent in the densely populated inner Auckland and CBD area would result in the greatest population required to evacuate, and also the greatest need for public transport support, given the low vehicle ownership in this area.
"The next eruption in the Auckland Volcanic Field could happen anywhere in the existing field, either on the land, or in the sea, so estimating how many people might be impacted carries a lot of uncertainty," Wild explained.
"An eruption is unlikely in our lifetime but we know an eruption will occur in the Auckland Volcanic Field in the future. What's difficult is that we don't know where the next eruption is going to be."
Auckland Emergency Management (AEM) general manager Kate Crawford said an eruption was one of the region's "most impactful hazards and came with many risks.
"Evacuations themselves can be dangerous and asking people to leave their homes during an emergency, possibly for an extended period of time, is one of the hardest calls any emergency decision-maker will have to make," she said.
"That is why research to help us understand how much time we might have, and how long it will take to get people to safety is so valuable to us."
Once GNS Science advised AEM of a volcanic threat, the agency then began working with the police, which led Auckland's Evacuation Control Team and welfare and support agencies.
Unlike the big volcanoes in the central North Island, eruptions in the Auckland Volcanic Field are usually smaller, and in a new location each time.
There are currently 53 identified volcanic centres, which have created the cones and some lakes we see around the city.
"As the magma that feeds a new volcano gets closer the surface, it becomes easier for scientists to estimate where the new eruption will occur," explained AEM's principal science adviser, Dr Angela Doherty.
"But there would be a fine balance between waiting for clarity and leaving enough time for everyone to evacuate safely."
Dr Adrian Pittari, a senior lecturer at Waikato University, said the new study was a "useful overview" of the range of issues that would be faced should the risk of an eruption in Auckland be increased.
"The volcanic risk in Auckland is real; so is the uncertainty in the location of the next vent within the metropolitan area," he said.
"However, there are no signs of any imminent eruption or unrest in the deep subsurface, so the risk is no different to what it has been over the last few hundred years - which is currently low."
Wild said the next step in the research would be to develop maps and tools to support decision-making during volcanic crises.
"We have seen from past events around the world that the time it takes for decision-makers to analyse the information and call an evacuation is the longest part of this process."
As part of the planning, Crawford said Aucklanders can help emergency services by creating a household plan ahead of emergencies.
"This should include where you might go if you had to evacuate your home, and what you might need to take with you if you were going to be away for a long time."
The study, funded by the Earthquake Commission and the DEVORA programme, followed another paper that explored how Auckland could recover from a simulated major eruption in South Auckland.
That found local sectors could face billions of dollars in costs as a result, and entire industrial and residential areas could be reshaped over two decades.
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