An emergency planning forum in New Plymouth has been told the biggest threat to public health from an eruption on Maunga Taranaki will be from disruption to infrastructure.
Natural hazard, risk and disaster researcher professor Tom Wilson of the University of Canterbury told a gathering of health sector experts there was a 30 to 50 percent chance of an eruption occurring on Maunga Taranaki in the next 50 years.
He said while pyroclastic flows, debris avalanches and raging mud lahars were a risk to individual safety, a greater threat lay elsewhere.
"The vast majority of water supply comes from draining rivers off the maunga, so in a future eruption some of that water could be contaminated by ashfall or even worse we could have lahars or some of those volcanic mudflows coming down and damaging - or even destroying - some of those water supply systems, which has very severe knock-on effects for communities and farms that are reliant on that water."
Wilson said that same ashfall could knock out electricity supply into Taranaki, plunging the region into darkness.
"In the case of Taranaki, we have one entry point for the national grid or the electricity transmission network coming into the region. That's all at Stratford, which is 10 to 12 kilometres downwind from Taranaki Maunga.
"So, there's a really good chance it will get ashfall and could be disrupted if there's enough ash on that electrical equipment to cause a flashover or a short circuit which could put the region dark, which is a scary prospect."
Wilson said lahars - which Taranaki was famous for - could also take out bridges, pipelines and other infrastructure leaving people isolated.
"I guess close to where the eruption occurs they'll be really energetic volcanic processes where that will be a life safety risk for people there, so again evacuations certainly within the park boundary will be really important and depending on how big that eruption might be that would be something Taranaki Emergency Management would be really seriously thinking about."
Associate professor Carol Stewart of Massey University - an expert in disaster environmental health - told the meeting those very same evacuations brought public health challenges of their own.
She had studied the aftermath of the 2015 Calbuco eruption in Chile, where more than 7000 people were evacuated, 4000 of whom were moved into shelters.
Stewart said while nobody died as a result of the eruption or its immediate aftermath, there were elevated levels of respiratory and gastro illness.
"We know that any event that displaces people from their homes and anything that forces people to live in shelters and evacuation centres and that sort of thing carries increased health risks."
People affected by the eruption also suffered psychologically.
Stewart had also looked into the effect of ash on people in eruption zones.
"What we found was generally speaking, healthy people can tolerate relatively high levels of ash in the air for short periods of time. The groups that are more at risk are those with any pre-existing respiratory conditions, so people with conditions such as asthma, chronic obstructive pulmonary disease, chronic bronchitis and so on, and they can be quite vulnerable."
She said the general advice when ash was falling was to stay inside with doors and windows shut, and if you had to be outside, wear a mask.
'It's about being ready'
Co-emergency management lead at Te Whatu Ora Taranaki, Cameron Grant-Fargie, said the forum was a wake up call for organisations and individuals to be prepared for the inevitability of an eruption on Maunga Taranaki.
"I think with the greater understanding of the science we are now looking to move closer to the community and have all the smaller businesses and healthcare facilities being more ready, so that's our aged residential care people, our general practices, our non-governmental organisations.
"It's about more than evacuation because for an evacuation you've got to have somewhere to go. It's about being ready and having enough food, having a water plan and also knowing how your staff are going to respond."
Cameron Grant-Fargie said organisations needed to reach out to emergency management agencies now, and have a plan in place should the worst happen - not throw one together when it was too late and an eruption was upon them.