The initial tremors will be too small to notice. Aucklanders won't feel the tiny quakes, caused by hot magma driving its way upwards from 100km below the city.
Those first warning signs will be detected by sensors in deep bore holes, and sent to experts at GNS Science in Wairakei, near Taupo. If magma is on the move, the region's 1.4 million residents have days, maybe weeks, to prepare for a volcano to blow.
A massive emergency response will be triggered: Civil Defence, police, fire, defence forces, Government agencies, and scientific advisers. But it is not until the red-hot magma is close to the surface, causing the ground to bulge, that they will know who to evacuate. Auckland's volcanoes are monogenetic: they never blow in the same place twice. It's not a hypothetical scenario, says Greg Holland, geologist and Auckland Regional Council hazards manager.
The volcanic field will become active again, "we just don't know where, when, how big, or for how long." There's a chance the next one will be smaller than the last the eruption that formed Rangitoto 800 years ago but even a minor blast would be catastrophic in a densely populated city. "The implications are so much bigger, not just for Auckland but for the rest of the country," Holland says.
"Nowhere else has a volcano like this, with a million people sitting on top of it." A volcanic eruption in Auckland in the next 50 years has about a 4 per cent likelihood, although "considerable uncertainty surrounds this figure," according to the Ministry of Civil Defence and Emergency Management.
Other disasters are still more likely: a Government briefing paper says there's a 20 per cent chance of a major earthquake on the South Island alpine fault in the next 20 years, a 30 to 50 per cent chance of an ash-producing eruption from Taranaki within 50 years, and a 15 per cent chance of a major shake in the capital during the next half century.
Emergency plans, warning systems and scientific advisory groups are in place, but preparation can go only so far. The public message is this: prepare to look after yourself, because we might not be able to.
"It's a community and we have to do this together," says Rian van Schalkwyk, from Wellington's civil defence emergency management group. "That's the only way it's going to work."
Evan Harvey was driving when the first shake struck Edgecumbe in 1987. "You could see the waves coming along the road, about a foot high," he says. "The lamp posts were swaying from side to side."
Two more quakes struck within a few minutes, and several more would be felt over the next weeks. Roads were buckled, forcing Harvey to go by pushbike to reach his children at school. "My daughter was nearly hit by a netball hoop, and my son was under a table. He said it was just like driving dodgems, because they were moving all over the floor." At the bank where he worked, everything was tipped upside down.
At another bank nearby, the door had been wrenched off a three-tonne safe. Harvey said he didn't have emergency supplies stored then and he doesn't now, either.
Water was cut off, but supplies were tanked into the town quickly after the quake. "One thing it did do is bring the community together, because everybody started assisting everyone else."
But while Edgecumbe residents coped, Harvey thinks emergency services would struggle with a similar event in a big city. "When you've got highways, motorways and bridges down, they've got major difficulties. "In a big city you're going to have injuries and all the hospitals would be affected. I think they'd struggle, big time."
If a regional emergency is declared in Auckland tomorrow, Harry O'Rourke will be in charge. The Civil Defence Emergency Management Group controller will make the big calls on whether you leave your home or stay put, where you go, how long you'll be away.
O'Rourke doesn't answer to mayors or politicians. Theoretically, he can over-rule even police chiefs, he says, "but why would you, when they're the experts?" Once scientists say an eruption is on the way, he will requisition buses, trains and boats to move people out of the danger zone. Waikato and Bay of Plenty authorities will be asked to prepare to house and feed evacuees.
At Auckland Zoo, animals will be moved inside, and food and medication stockpiled. In a worst-case scenario, vets need to be ready to euthanase animals. Staff with gun licences will remain on site for that last resort. When the volcano goes up, there'll be ash, fires, gases, and lava flows.
The "devastation zone" is expected to extend for a 3km radius around the volcano; people will be evacuated within 5km. New Zealand has seen disasters before, but nothing like this. Emergency services will have to work in a whole new way. "The big one will be something quite different," O'Rourke says.
Take Taranaki, which last erupted about 250 years ago. If it blows again, local civil defence emergency management group planning assumes ash fall could be swept east across the country, reaching Napier, Hastings and Gisborne. Most of Taranaki would be at risk of fast-moving mud flows, and Opunake, Eltham, Stratford and Inglewood could potentially be affected by lahars and debris. If Auckland and Taranaki are at the mercy of volcanoes, Wellington is held to ransom by a fault line.
A major earthquake would be catastrophic for the capital, yet, like an Auckland eruption, it's just a matter of time. Emergency plans are based on a shallow quake measuring around 7.6 on the Richter scale. "We are well within the return period for this earthquake," says van Schalkwyk.
What we know: hundreds of buildings in the capital's CBD will collapse, leaving people trapped under tonnes of concrete, glass and steel – a search-and-rescue scenario far beyond local capabilities. "We'll have to rely on international assistance, and those forces will be three or four days away," van Schalkwyk says.
Roads will be blocked, paramedics will look for survivors on foot, carrying backpacks of medical supplies. Fire trucks won't be able to reach fires or access water for firefighting, as emergency valves will shut off to prevent water gushing through thousands of pipe breaks.
Landslides will probably leave the capital isolated (the motorway is built on the fault line) and even within the city, small pockets will be cut off. Triage points will be set up to deal with the seriously injured but walking wounded will have to fend for themselves. Some 50,000 people will be homeless, and there may be no means to evacuate them. Residents will need to look after themselves and their family first, then take others in where they can.
Water will be critical: "It could take up to 75 to 80 days to repair those bulk [water] lines," van Schalkwyk says. "That's why we're driving the campaign that people need to store water and food."
There are signs the Get Ready Get Thru campaign, which encourages people to have an emergency plan and supplies stored, is making progress. But there's plenty of work to do, especially in Auckland. A survey to be released next week is expected to show the number of Aucklanders who are fully prepared has risen from 3 per cent to just 7 per cent.
Dr David Johnston, from Massey University's Joint Centre for Disaster Research, says part of the problem is we've had it so good for so long. "It's difficult to motivate people to do things that they can't relate to," he says. "New Zealand has had extremely good fortune to have had 70 years without a major natural disaster.
Sooner or later events will occur that will hit our urban areas that will remind us where we are." Studies have also shown that some think a major event will be so devastating that getting ready won't help. But Johnston says there are few outcomes from a disaster that can't be anticipated and planned for. "With Cyclone Katrina, it followed almost a textbook example of the scenario that was outlined," he says. "The research community and even the planners at the government agencies knew exactly what was going to follow, and it went how they predicted."
A major tsunami last struck New Zealand in 1960, flooding Gisborne, following an earthquake off the coast of Chile. Experts are still looking to South America for our next big wave. The Pacific Tsunami Warning Centre based in Hawaii gives an early alert when a quake occurs, and in the case of a South American tsunami, that would mean 10 to 12 hours' warning. But with a closer event, like the Tongan earthquake that sparked a tsunami advisory in March, there will only be about three hours to prepare.
The response from local authorities to that threat has highlighted the need for a national warning system. O'Rourke activated the Auckland region emergency operations centre, but says other local authorities didn't. Had the threat been real, they would have had little time to react once the tsunami was confirmed. "We need a national warning system, and we need it soon," O'Rourke says. Though experts don't expect anything as devastating as the 2004 Boxing Day disaster in Asia, even a surge of a few metres could swamp key roads and motorways.
Policies for evacuating coastal areas differ widely sirens will activate in Waitakere and Rodney, and an automated phone alert system will notify North Shore residents. Holland says there needs to be a "whole suite" of warning systems to make sure the message gets out. "It's also about people who live on the coast taking ownership of what their risk is." Last year's practice run for an Auckland eruption, Exercise Ruau-moko, showed a volcano blowing its top in our biggest city could wipe 14 per cent from New Zealand's GDP.
In Wellington, a major quake could trigger property claims totalling $6.2 billion. The good news is we're insured – the Earthquake Commission (EQC) has a $5.5 billion fund and a further $3b in re-insurance, its own insurance policy against a catastrophic disaster. And the Government also guarantees EQC will meet its obligations in any event, so claims over $8.5 billion would continue to be paid.
Insurance manager Lance Dixon says so far the biggest payout has been for the 1987 Edgecumbe earthquake, which cost $135m in claims. A major disaster would see contract staff from New Zealand and Australia swing into action. After the Gisborne earthquake in 2007 a temporary local office was set up which, at its peak, was staffed by 75 people, processing $26m in claims. A national emergency has never been declared in New Zealand, but the National Crisis Management Centre under the Beehive is activated several times a year, often for severe weather events.
A duty team is on call at all times, as are scientific advisers from NIWA and GNS. But even in a national emergency, the most important response will be the local one, says public information manager Vince Cholewa. "What's happening in your home, your school, your workplace, that's the reality where you are. And the first response will always be the people right there, when it happens."