The scale of catastrophe facing Aotearoa in the case of a massive earthquake and tsunami from our largest fault - the Hikurangi subduction zone - has been laid bare by experts.

Tens of thousands of people could die, hundreds of thousands could be displaced, and the cost of building damage could top $100 billion.

Some of the country's top science minds have gathered at Te Papa to discuss the latest disaster risk, resilience and recovery science at a conference run by one of the government-funded National Science Challenges - the Resilience to Nature Challenge.

During a session on catastrophic risk, the National Emergency Management Agency (NEMA) detailed its work over the past 18 months, using a 9.1 Hikurangi earthquake and tsunami as its 'planning scenario'.

"Science tells us that a catastrophic event, such as an alpine fault earthquake, or a Hikurangi fault subduction zone earthquake and tsunami will very likely happen, if not in our lifetime, in our children's," said NEMA chief advisor to the chief executive Sarah Holland.

"It could happen tomorrow."

The fallout

If that level of earthquake hits, there would be four to eight minutes of shaking. For comparison, the ground shook for 10 seconds in the devastating 2011 Canterbury earthquake, Holland said.

Assuming 70 percent of people were able to evacuate, more than 22,000 would die - mostly in the tsunami - and nearly 26,000 more would be injured.

About 400,000 people would be displaced and 30,000 homes destroyed or damaged from the tsunami alone.

There would be about $144 billion worth of damage to buildings from the earthquake and tsunami.

And the scale of the damage to critical infrastructure is yet to be modelled.

"This class of catastrophic events is beyond our current arrangements, thinking, experience, and beyond our imagination," said Holland.

But preparation was underway.

The planning

NEMA is planning to respond to "the worst of the worst", working with 50 organisations to create a "catastrophic handbook" which can apply to any disaster, said Holland.

"That's things like mass casualty management, the provision of food, water and shelter, restorations of critical infrastructure, establishment of emergency supply chains."

International assistance would be vital for the latter, and NEMA was planning for what would be needed, and how it would be distributed upon arrival in New Zealand, she said.

Laying out the potential fallout was scary, but necessary, said NEMA chief science advisor Tom Wilson.

"These are big, big numbers," he said.

"This is what motivated change for chief executives. This is where elected officials really stood up and took notice."

But there was more work to be done in getting government and communities to properly understand the level of risk.

"We will be driven kicking and screaming into that space, by the insurance sector one way or another," he said.

"But we must have a better platform for being able to compare and characterise our risks, and understanding what those potential risk treatments or resilience strategies might be."

Science funding drying up

The science community was urged to keep their work coming.

But that would be tough, with 10-year funding for the National Science Challenges ending in June, said Massey University natural hazards professor Jon Procter.

Some areas of research would have to stop altogether, he said.

"The people doing planning and mitigation will be relying on old knowledge and old scenarios, rather than thinking about the next big thing or the next issue could occur," said Procter.

The National Science Challenges had brought experts together, said University of Canterbury lecturer Kristie-Lee Thomas, who also leads the Challenges' Whanake te Kura i Tawhiti Nui programme which applies mātauranga Māori to natural hazard research.

"Experts across different spaces, mātauranga, science, planning, policy," she said.

"You've built up these really good relationships, these really good teams... we want to continue working together and building up that momentum, but we need the resourcing to keep doing that and keep supporting that planning."

Invest in NEMA - inquiry

Just three weeks before the conference, a national inquiry into last year's weather disasters found Aotearoa was not prepared for large-scale emergencies.

In one of the inquiry's many findings, it noted NEMA is a small organisation that "does not have the funding or expertise to undertake the full breadth of activities it is currently tasked with".

The agency would operate more effectively if it focused on leading readiness and response activities - including setting standards, training, and leading worst-case scenario planning. The inquiry recommended it receive more cash to do so.

Meanwhile, NEMA's own probe into its response to the weather events is due on 16 May.

Main image (RNZ/Supplied/GNS Science): Tens of thousands of people could die, and hundreds of thousands could be displaced, if the Hikurangi subduction zone caused a massive earthquake.