Hundreds of thousands of North Island homes could be covered in ash in next volcanic eruption

Hundreds of thousands of North Island homes could be covered in ash after a volcanic eruption - and assessing the damage could take months, the Natural Hazards Commission says.

It has funded GNS to design a model that will enable authorities to quickly identify in real time where the ash is falling, and how much there is, to help emergency managers respond.

Mount Ruapehu last erupted in 1995 and 1996.

GNS natural hazard and risk scientist Dr Josh Hayes, who is helping to create the model, said while those eruptions were significant, there was not much damage - they were more of a nuisance.

But Ruapehu would erupt again, he said.

"Let's ... think the next 50 years, I'd say an eruption of some size at Ruapehu is almost certain to occur within that 50-year time period.

"What's really uncertain is how big it would be, and what areas would get affected."

Because the country had never seen major ashfall, researchers did not have much data to forecast potential impacts - relying instead on overseas findings, Hayes said.

But the GNS model would quickly identify how widespread the ash fall would be during an eruption, to inform emergency response teams about which homes and other buildings may be affected.

"We'd use ashfall forecasts that come through Geonet, and then we would tie that in with some building data that we have, around where buildings are located, and with the vulnerability models, which would then give us an indication of what the likelihood is of different buildings suffering different types of damage. "

The tool would be constantly adjusted with fresh data, which was critical if a volcano continued erupting, he said.

It was unlikely that damage to buildings would be too major - ash could accumulate in gutters which could collapse, for example.

That was partially because national parks surrounded most of the country's volcanoes, Hayes said.

"This sort of acts as a bit of a natural buffer.

"Because we don't have a lot of development, a lot of buildings, located near volcanoes, it means that widespread building collapse, like the really significant damage, is probably very rare."

The work is funded by the Natural Hazards Commission. Its head of research Dr Natalie Balfour said it was one of a handful of projects helping forecast the impacts of volcanic hazards, with others looking at volcanic gases and lava flows.

"All of our investment and research at the Natural Hazards Commission is trying to help make Aotearoa New Zealand more resilient, so how can we help better prepare people by understanding what the impacts of the natural hazards might be on them and the properties."

The ash fall model would help communities recover and get back on their feet more quickly, she said.

Ruapehu mayor Weston Kirton said he remembered the 1996 eruption well - he was the mayor back then, too.

"I was actually there the morning it happened, on 17 June 1996, it happened to be my birthday," he said.

Kirton was up on the mountain with his daughter, who was training to work on the ski field.

"The mountain erupted, so I was right on the job," he said.

While scientists considered the ash fall moderate, Kirton said it caused havoc - closing ski fields and airports as far away as Auckland, ruining car filters, iron roofs, and the local economy, and creating a big clean-up job.

"Everyone was evacuated from the area, but it was such a horrific experience to be there, and witnessing it was just nerve-wracking.

"Of course everything unfolded from there ... emergency services came, road closures, and the dust and ash was all round the country."

The Ruapehu district was always on high alert for volcanic activity, and anything to help plan and respond quickly - like the ash fall model - would be gratefully received, he said.

The model is due to be ready by the end of 2025.