A "swarm" of earthquakes somewhere in Auckland is expected to be the first warning of the birth of a new volcano in the region - a catastrophe that could destroy an area 5km wide with lava, ash and gases super-heated to more than 200C.
No-one knows when the disaster will hit, how big it will be, or where within the Auckland Volcanic Field the new vent through the Earth's crust will open.
But scientists from GNS Science and Canterbury and Massey Universities, detailing what they call a credible eruption scenario, say the Auckland Volcanic Field "will almost certainly erupt again".
Their doomsday scenario, described in the Journal of Volcanology and Geothermal Research, is centred in the shallows of the Manukau Harbour, just off suburban Mangere Bridge and plays out over two months.
It contains a long list of volcanic threats: large earthquakes; ash falling throughout Auckland from a 7km-high column; flying blocks of volcanic material; lava flows; a mini-tsunami; huge explosions when hot, high-pressure lava passes up through the earth's crust and vaporises cold seawater; and the devastation of high-speed, high-temperature "pyroclastic" clouds of gases and fragments.
Dr Natalia Deligne, of GNS Science, and colleagues note that former prime minister John Key's Department of Prime Minister and Cabinet in 2011 "deemed an Auckland eruption to have 'major' to 'catastrophic' consequences for New Zealand, a consequence considered to be similar in magnitude to a gobal conflict or human pandemic".
Auckland City is built on the 360sq km volcanic field, which has more than 50 vents. Volcanoes have erupted in the field more than 55 times in the past 190,000 years.
The authors say scientists have been unable to identify any patterns in the eruptions.
"In fact, the oldest vent, [Takapuna's Lake] Pupuke, and the youngest, Rangitoto [Island] are located next to each other."
"The most recent eruption, Rangitoto, accounts for nearly half of the erupted volume of the field, and it is unclear if this eruption was an anomaly or signals a change in the eruptive behaviour of the field."
The pyroclastic surges from the Mangere Bridge eruption would be the most damaging feature of the new volcano's blasting into life.
"The first pyroclastic surge causes surface damage ... complete destruction within 2.5km of the vent, severe damage to most structures and destruction of weaker structures from 2.5-4km, and some damage to weaker structures from 4-6km."
Much of Mangere Bridge suburb would be buried in 2m of material from just the first of the three pyroclastic surges.
A lava flow would close off the Mangere Inlet, creating a new lake. A section of lava breaks off, causing a 2m-high tsunami to surge around the harbour.
Following the eruptions, the harbour would be left with a cinder cone 900m wide at its base and with a tuff ring of 1.2km, the scientists say.
The research is part of a series of studies of the impact of an Auckland eruption on electricity, transport and other infrastructure.
Infrastructure and evacuation
The scientists estimated the impact of their hypothetical eruption:
• 7 weeks - evacuation of surrounding areas
• 6 months-plus - clean-up
• 1 year-plus - electricity outages
• 7 weeks-plus - road outages
• 1 year-plus - wide-scale water restrictions
• 2 years-plus - raw sewage discharges
• 2 weeks-plus - telecommunications outages