"We just had no idea it was going to happen so soon," said Bill Albert.
Sarah Crump, 30km away in National Park, felt a thump just before midnight.
Dave Allan, whose nearby house was swiftly painted grey, heard a "deep boom".
Malcolm Haitana described a noise like the roar of a jet engine, and a beautiful flashing plume climbing kilometres into the moonlit sky, then showering the landscape with muck for 2km around.
At a press conference the same morning, GNS vulcanologist Brad Scott spoke of a volcano which sneaked up on scientists and roared into life without warning.
There couldn't have been a better example of the uncertainty even the best science faces when predicting eruptions.
In an interview with the Weekend Herald last month, two weeks after a series of tremors below the eruption site pushed alert levels up, Mr Scott recalled how the 10 months before Mt Ruapehu's 1995 eruption also yielded few signs it was priming to blow.
"Even today, looking back, there's nothing that races out of the woodwork to say that Mt Ruapehu's biggest eruption in 50 years was going to kick off."
Devastating earthquakes and tsunamis have been more visible in headlines than volcanoes in the past few years, yet the ever-present volcanic threat is just as real.
New Zealand, spread across two plates and on the Pacific Ocean's explosive Ring of Fire, is home to a dozen volcanic sites under regular watch.
Three have alert levels above zero, and two are displaying minor eruptive activity.
Each second, data from seismic sensors throughout the country is transmitted to GNS Science's Wairakei Research Centre.
Patterns which differ from baseline data are closely analysed and, as happened on Mt Tongariro last month, teams are sent to the site to test the geochemistry.
GNS is New Zealand's principal watchdog on volcanic activity, and has the authority to raise alert levels at any moment, setting off a seriesof actions which could includemass evacuations and diversion of flights.
But the timing of quick-fire eruptions such as Tongariro's on Monday is notoriously hard to pick, even after the warning flags have gone up.
"A lot of what we try to do is emphasise that uncertainty," GNS head of vulcanology Gill Jolly said.
"It's just a case of this is one scenario and there may be other scenarios that go in a totally different direction.
"We can forecast a 20 per cent chance of an eruption happening tomorrow, but we can't say 2pm on Tuesday."
Minor eruptions within the volatile Taupo volcanic zone - as also seen at Mt Ruapehu five years ago - could happen with little warning each decade.
Luckily, the biggest bangs we could face - rare and extreme caldera eruptions - would be unlikely without months,or years, of notice.
"One of the difficulties, however, is none of these huge eruptions have been monitored in historic times, so we can only really speculate about what we might see," Dr Jolly said.
"We certainly would expect a lot more ground deformation and gas, but until it happens we just don't know."
Our best-known example, the Hatepe or Taupo eruption around 1800 years ago, would have created effects visible in China and Rome and fired a devastating 1.5km-high pyroclastic flow that covered the landscape with ash and pumice for 80km around.
A Bay of Plenty scenario described in a 2005 Civil Defence report, serious enough to involve a pyroclastic flow of 10km to 15km, lists effects as deaths, injuries in less severely affected areas, ash-laden roofs collapsing and vegetation and buildings igniting.
Add to that the chaos caused by food and medical supplies being cut off, ash polluting household water tanks and clogging vehicle fuel filters and power blackouts.
"I think it's very unlikely in my lifetime that I'll see something on the scale of a Taupo eruption ... and I'm glad about that," Dr Jolly said.
It was more possible the next 50 years could bring worrying but benign signs of unrest in one of our three caldera volcanoes - Taupo, Mayor Island and Okataina, whose most recent display was the 1886 Mt Tarawera eruption which killed 153 people and buried the Pink and White Terraces.
"And eruptions come in an enormous range of sizes," she said.
"Of the 26 or 27 eruptions in Taupo over the last 23,000 years," Mr Scott said, "you could have watched 10 or 12 of those from a waterfront cafe ... but three of them could have been watched from a cafe in Sydney."
He was sure of seeing a significant eruption on his watch at rowdier White Island, whose minor eruptions this week were the first in a decade, at Mt Tongariro, which is also carrying an alert level of two, or at neighbouring Mt Ruapehu, also showing signs of unrest at Level 1.
Of our sleeping volcanoes most likely to wake next, Mr Scott has his bets on Mt Taranaki, but its first bang since 1775 could still be hundreds of years away.
The threat it posed was more worrying, he said - more than 85,000 people live within 30km of the mountain today, 40,000 in high-priority evacuation areas.
The full volcanic arsenal was there - lahars gushing down the mountain's watercourses, pyroclastic flows and sector collapses spreading up to 20km from the vent, lava flows reaching up to 10km and a near-certain ashfall threatening our dairy hinterland and source of natural gas.
Just as frightening would be the toll of even a small, localised eruption in the Auckland volcanic field, home to almost 50 volcanic centres and around 530,000 people.
Scenario plans assume buildings and infrastructure within 3km of an erupting vent would be destroyed by an initial surge of hot gas, steam and rocks and ash would fall over most of the greater Auckland area - up to 10cm thick near the vent.
Ash and acid rain would pollute water supplies and probably destroy sewerage infrastructure.
Auckland International Airport would be closed for weeks and insured losses could be in the order of $1 billion to $2 billion.
None of Auckland's existing volcanoes were likely to erupt again, Mr Scott said, but the Auckland field was geologically young and potentially active.
GeoNet began upgrading seismic monitoring technology of the field in 2006, increasing the number of stations and sites with sensors in deep boreholes.
But although the field was monitored to detect magma movement within the earth's crust, the location of the next vent and the area which would have to be evacuated might not be known until an eruption was imminent.
"The science of it all is fantastic to be involved in," Dr Jolly said, "but in terms of the responsibilities we have, the stress levels can just peak in some ways."
How often did she imagine the worst?
"Every time I switch on my computer in the morning and look at what the data is telling you.
"But if you got too scared constantly thinking of a Taupo-style eruption, you wouldn't be able to do your job properly," she said.
"You just have to be mindful all the time."
If there was any message scientists wanted to push, Mr Scott said, it was to have an emergency plan and a survival kit.
"You just never know when things are going to change."
For information on preparedness, visit www.getthru.govt.nz
Guide to our hot spots
A quick look at New Zealand's most volatile or potentially devastating volcanoes
White Island (Whakaari)
May have just entered new eruptive cycle after a series of eruptions since Sunday. Has had three eruptive cycles since 1976. The uninhabited island is the visible tip of a 750m high, 17km wide submerged volcano, New Zealand's most active. Produces lava flows, minor ashfalls and its crater has collapsed several times. Small amounts of ash were reported to have reached Papamoa on Thursday night. There had previously been no evidence of erupted material reaching the mainland, although research suggests the volcano is capable of producing large eruptions.
Last eruption: Erupting now.
Mt Tongariro
Dormant until this year, the volcano produced a large eruption in 1869 which formed the upper Te Mari Crater. Another eruption 23 years later belched an immense quantity of steam, mud and boulders, and ejected material rose 600m-900m before rushing down the mountain side. Scientists are investigating whether a steam-driven eruption on Monday night, affecting a small isolated area, will be followed by more.
Last eruption: Monday, August 6, 2012.
Mt Ruapehu
New Zealand's largest cone volcano has mostly produced lava and ash in its frequent eruptions. The last explosive eruption lasted seven minutes and spread ash, rocks and water across the summit area, producing lahars in two valleys including one in the Whakapapa ski field. In contrast with the previous eruptions in 1996, there was no high ash plume to produce ash fallout over a wide area. The effects of an eruption on local tourism, particularly skiing, is large and ash can spread over large areas, especially toward the east in prevailing westerly winds.
Last eruption: September 25, 2007.
Mt Ngauruhoe
Discharged red-hot rocks of lava and periodic activity that lasted for months when it erupted in 1973. In the next two years, there were explosive eruptions of ash, and blocks of lava were thrown as far as 3km. During the last violent eruption, gases streamed from the crater for several hours, producing a churning plume of ash up to 13km above the crater. This column then collapsed, causing ash and scoria avalanches that swept down the sides of Ngauruhoe, leaving trails of rubble.
Last eruption: 1973-1975.
Mt Taranaki
More than 85,000 live within 30km of Mt Taranaki - 40,000 in high-priority evacuation areas. Risk of lahar flows, pyroclastic flows (up to 20km from eruption vent), lava flows (up to 10km) and ashfall. Could disrupt dairy farming and petrochemical industries, including reticulated supply to North Island.
Last eruption: 237 years ago.
Taupo Volcano
Has not erupted since human settlement of New Zealand, but has been one of the most active caldera volcanoes on Earth over 300,000 years. Largest known eruption expelled more than 500sq km of lava, ash, rocks and gas. Effect of even a small eruption could be devastating for the central North Island and its effects would be felt throughout the entire country. As well as direct damage, it would inflict severe losses on tourism, agriculture, forestry and North Island hydroelectric generation industries.
Last eruption: 1800 years ago.
Auckland Volcanic Field
Home to nearly 50 volcanic centres. Mass evacuation for an unknown length of time would be essential. Planning scenarios include an ashfall over Greater Auckland, significant damage to infrastructure, airport closure and insured losses of up to $2 billion. Existing volcanoes unlikely to erupt again, but field is young and potentially active.
Last eruption: Rangitoto, 600 to 700 years ago.
Bay of Islands and Whangarei volcanic fields
Bay of Islands volcanic field contains 30 vents, mostly comprising scoria cones and lava flows and domes. Little is known about the field but is likely to have erupted 10 times in the past 20,000 years. Its area is not heavily populated but Bay of Islands is a popular tourist destination. Whangarei volcanic field's last eruption included small eruptions of ash, scoria and lava.
Last eruptions: Bay of Islands Volcanic Field: 1300-1800 years ago.
Whangarei Volcanic Field: 250,000 years ago.
Source: GNS Science, National Hazardscape Report
