Scientists say a lahar caused by Ruapehu's Crater Lake spilling its banks could reach a size three times the size of the disastrous wave in the 1953 Tangiwai disaster.
The chance of a lahar occurring within the next eight weeks is placed at somewhere between 30 per cent and 40 per cent so the risks are high.
But while it could happen any day, Department of Conservation scientist Harry Keys says there is a better chance it will come next summer.
In the worst-case scenario, the lahar could be up to three times bigger than that of 1953, although the more likely one is that it will be about the same size or a little larger.
In the path of destruction are the main trunk line's rail bridge, State Highway 49's traffic bridge, several local bridges, fibre optic cables and two of Transpower's national grid transmission pylons. As well as, the Tangiwai memorial marking the loss of 151 lives and the rebuilt Tangiwai bridge across the Whangaehu River.
Digger and bulldozer operators worked busily this week to shift volcanic ash, rock and debris from beneath the rail bridge, creating a wider berth for the predicted lahar.
In the upper reaches of the river, Dr Keys says the debris will flow at an average speed of 20km/h, taking about two hours to get to Tangiwai.
From this point it will slow significantly, eventually reaching the Tasman Sea 100km away, about 22 hours later.
"The highest risk of damage is at Tangiwai," Dr Keys says.
At the top of Mt Ruapehu, hard volcanic rock surrounds the crater at a minimum altitude of 2529.3m, but the lake level is more than 6m higher than this (2535.4m).
The high lake level is the result of the natural build-up of a temporary dam. During the volcanic eruptions of 1995 and 1996, a barrier of scoria and ash built up the side of the crater, forming an unstable, weak and permeable dam.
The body of water creeping up the side of the dam is filling at a rate of about 10,000cu m a day, thereby raising the lake by 4cm daily.
High summer temperatures produce greater ice and snow melt and this, with high rainfall, exacerbates the situation.
If the dam holds, spillage will inevitably occur at the barrier's maximum height of 2536.9m, but scientists are certain the dam will burst its banks when the water level reaches 2536.5m.
When the collapse comes, it will plunge an expected 1.2 million cu m of volcanic ash, rock and water down the narrow Whangaehu River bed, creating the kind of scene not witnessed since the 1953 disaster.
The lahar could be fronted by a bow wave of about 2m.
Despite nine years of preparation for this event and a sophisticated early-warning detection system, it seems memories of the 1953 Tangiwai rail disaster are still raw.
The digger and bulldozer workers beneath the rebuilt bridge this week were under strict instructions to report their safety regularly to their bosses in Wellington.
"I'm ringing up every half-hour to the Wellington Railway Station, which is the central control centre," said site supervisor Bart Venema.
"We've got to check in so they can make sure we are still here."
Indicating how imminent the threat now is, the heavy machinery has been operating at the site from 7am to 6pm every day this week.
But Mr Venema was confident of leaving the area without incident.
"It [the lahar] will take about 1 hour to get here. The alarm will go off and they'll ring me and they'll cut off access to the bridge and the area will be evacuated."
In 2005, a $5 million project by Transit to raise State Highway 49's bridge across the riverbed was completed. The bridge was raised 2m, new piles were driven into the ground and extensions were made to existing piers to make them stronger.
Before that, response plans were developed by Ruapehu and Taupo District Councils for the southern and northern areas likely to be affected.
A 300m bund was built on the eastern side of the mountain in February 2002. It stands 4.6m high and 20m wide and is made of gravel, ash and boulders sourced from the surrounding lahar flood plain.
Its purpose is to prevent overflow into the Waikato Stream and Tongariro River from the Whangaehu River channel. Genesis Energy will be on standby to shut down its power plant at Rangipo should this happen.
Experts say the chances of the Tongariro and Lake Taupo becoming contaminated by the lahar are long shots, although the existence of the bund indicates the concerns are real.
In 2003, the Eastern Ruapehu Lahar Alarm and Warning System (Erlaws) was installed, consisting of three separate sensors at the Crater Lake, the NZ Alpine Hut and near the Tukino Skifield.
Erlaws will automatically warn road users of the lahar. Flashing lights and signs will activate on SH1 and SH49 and barrier arms will close the road bridge at Tangiwai.
Data from this system will also trigger an automatic alarm via a paging system. Messages will be sent to police, district and regional council staff, Transit, Toll Rail and scientists.
Farmers around Tangiwai will also be notified of the torrent.
Hamish Blackburn runs his 700ha family farm in the area which is divided by the Whangaehu River.
One of the council-owned bridges that provide access to his home is directly in the lahar's path.
This week Mr Blackburn had a call from Dr Keys warning him of the heightened state of alert and immediately moved more than 2500 sheep from the farm's flood plains.
"I don't think it will rise up to the fields but you can't be too sure," Mr Blackburn says from the bridge as he stares down at the rapidly flowing acidic waters below.
"My grandfather was here at the time of Tangiwai disaster. He was blocked in here for yonks. Bodies and debris were scattered through here, tied up in the banks up there. He never talks about it, it cuts too deep."
The DoC scientists, particularly Dr Keys, and Ruapehu Mayor Sue Morris have kept his family briefed.
"I went to a meeting last week. The dam itself is starting to erode."
Work on an impending lahar began after the eruptions of 1995 and 1996, when the last lahars produced headaches for DoC staff, ski and tourism operators and territorial authorities. Unlike the lahars of 10 years ago, however, this next one will not be triggered by volcanic activity.
It will be similar in nature to that of 1953. Such lahars usually happen every 20 to 50 years.
Discussion on how best to cope with the lahar has been controversial. Reports on the environmental effects were drafted in 1998 and 1999 and nearly 50 submissions followed public consultation.
Conservation Minister Chris Carter discounted engineering and human intervention options on the crater rim and Opposition MPs accused the Government of putting lives in jeopardy.
They claimed that political correctness and sensitivity over Maori cultural values were behind the refusal to intervene with engineering works.
But Mr Carter said other groups such as the conservation authority and Forest and Bird were also opposed to human intervention.
Co-ordination, communication, leadership and lines of responsibility are likely to become clouded when the lahar strikes. Whatever the outcome, there is no doubt more lessons will be learned.
Agencies involved in the Southern Response Plan for the main area to be affected by the lahar expected to flow down the Whangaehu Valley: Ruapehu District Council, Horizons (regional council), Wanganui District Council, Rangitikei District Council, Genesis Energy, Justice Department (Rangipo prison), Winstone Pulp International (pulp mill), NZ Army, Civil Defence, Fire Service, Ambulance, Search and Rescue, Police, Transit NZ, Works Consultancy, Opus, Good Health Wanganui, Department of Conservation, Geological and Nuclear Sciences, Transpower, Toll (rail), Genesis Energy (power station).
Tranquil scene gives few hints of danger
A wisp of volcanic steam rises from the Crater Lake. It coils through the wooden measuring stakes planted in the lake's soft dam and floats off above the Whangaehu River.
The river sparkles under a clear sky and curves off to the south beside the Desert Road then out of my view, to Tangiwai and the coast.
I am perched near the top of Mt Ruapehu, sheltering from the bone-chilling wind beside tiny Dome Shelter. With a photographer, I have trekked up through crisp morning snow from a hut high on Whakapapa skifield to see the lake that could send a torrent of mud and boulders bursting down the mountain's east side when its weak dam breaks.
No sign of a lahar today. Just the warning signs. "... lahar can happen at any time," says one at the shelter.
I peer at the dam of volcanic ash and rock through binoculars from the opposite side of the 500m-wide lake. The dam, plugging a gap in the south of the crater rim, varies in thickness from more than 100m at its base on rock to around 30m on top. It is nearly 7m tall, although I can see just the 1.5m above the water - a black wafer of trouble in my viewfinder.
When the dam inevitably breaks, it will uncork the 1.5 million cubic metres of water it is holding suspended 2.5km above sea level.
Threats aside though, the lake looks grey and harmless. The wind is fresh and I can detect none of the sulphur smell that can come with a visit to the crater. A little steam wafts off the lake's surface, only faintly suggestive of the power in this volcano's throat.
If I glance to my right I can see into the dark depths of a 2m-wide crevasse in the glacier which flows slowly into the lake. This curved crevasse is 80m long and 40m from the lake. Another, shorter one has cracked the ice nearer the lake's edge, where the glacier, more than 60m thick, ends abruptly in a vertical, ash-stained wall. Lumps of ice the size of a car have slumped, ready to totter and fall into the lake. Whole sections of the wall can collapse at once, causing waves that could break the dam.
As we set off back downhill, the first of the day's crater walkers arrive from the chairlifts about two hours below.
Like those who follow, Pam and Len Rodenburg from north Taranaki - with their children Billy, 13, and Katie, 11 - are curious about all the lahar talk and want to see for themselves.
- Martin Johnston