KEY POINTS:
It is only a matter of time until up to one million cubic metres of water - enough to fill 400 Olympic-sized swimming pools - will burst from the Mt Ruapehu crater lake and sweep down the Whangaehu Valley, says the Department of Conservation.
"If that water comes out in half an hour, you can expect quite a sizeable lahar," said DoC scientist Harry Keys.
Local authorities have been watching the lake for years and are prepared for the lahar threat, which is not expected to endanger residential areas.
Dr Keys said the lake was at a record level, 2.8m below the top of a dam that has started leaking up to 10 litres a second. The seepage is eroding the dam, which blocks the lake's usual outlet.
"If the lake keeps rising, the pressure will increase and a lahar is likely to happen sooner rather than later," Dr Keys said.
"In two weeks it might be ready to go, but it's more likely to be February or March.
"If it hasn't gone by then and level is dropping, we can be almost sure it won't happen till next summer."
Once the dam breaks, the lahar will cascade down the valley on the eastern side of the mountain to Tangiwai and out to sea.
A barrier of boulders and gravel 300m long, 20m wide and up to 4.6m high has been built to divert the lahar into the Whangaehu River, and away from the the Waikato Stream and Tongariro River.
The Whangaehu River could rise by as much as 6m, but this is not expected to threaten its banks.
"There are no houses in the way, and no one living in the path of the lahar," Dr Keys said.
"If it keeps rising for another month, then at least one bridge might be at risk, but not the main [State Highway 49] bridge." That bridge has been strengthened.
The Government has ruled out breaking the dam because of technical, safety and conservation concerns.
Lahar and eruption detection systems have been set up on the slopes of the Whangaehu Valley to warn police and local authorities. Residents would then be warned, and bridges and roads would be closed.
Dr Keys said authorities were more prepared than in 1953, when a lahar led to the Tangiwai rail disaster that killed 151 people: "Back then there was no warning system, the bridges were weak, there was no readiness at all."
