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Scientists used a professional kayaker on Mt Ruapehu's crater lake to install sensors to measure the rate at which the lake level falls if the tephra "dam" on the crater rim collapses.
They also installed a fixed digital camera overlooking the dam of soft volcanic material - sitting on the hard rock of the crater rim - to record its erosion by the crater waters.
The Department of Conservation (DOC) has said that a break-out is likely to occur by March and that it is highly probable the dam will fail.
If the water in the crater lake reaches the top of the tephra dam before it bursts, it will spill 1.5 million cubic metres into the Whangaehu River, triggering a fast-moving type of landslide known as a lahar.
On Christmas Eve 1953, such a lahar caused the Tangiwai rail disaster by sweeping away the main trunk rail line bridge over the river and 151 people on an overnight train died.
This year's lahar is expected to be twice the volume of the one in 1953.
DOC predicts the lahar in the Whangaehu River will travel at an average speed of 21km/h and take two hours to reach the road and rail bridges at Tangiwai.
Some of the scientists monitoring the crater lake have been hoping for a cool wet summer because that will make it more likely the crater will fill quickly and burst.
But recently so much water - over 5000 cubic metres a day - has been seeping out of the soft deposits of ash and pumice that make up the tephra dam, the level has been falling by 3cm a day.
Measurements last week showed it nearly 5m above the hard rock rim - a lahar at this level would be similar in size to the 1953 and 1975 lahars.
Scientists say the lahar is most likely to occur once the lake level rises another 2m.
DOC director general Al Morrison said yesterday a short-term solution - such as cutting a channel in the crater rim to bleed off the accumulating water - could increase the risk of a bigger lahar if the channel clogged.
"The idea that a simple, cheap solution will suffice is wrong," he said in an article in the Dominion Post newspaper.
- NZPA
