Monitoring at Mt Ruapehu's Crater Lake is being stepped up as authorities brace for a lahar to burst down the mountain's eastern slopes.
Staff of the Department of Conservation and the Institute of Geological and Nuclear Sciences are planning to revisit the volcanic lake either tomorrow or Friday, after a visit on Saturday found lake levels had swelled to within 1.5m of its unstable lip.
"The weather's looking possible for this Wednesday and not too bad for Friday," said DoC spokesman Dave Wakelin.
"But mountain weather changes so rapidly on you. Very often we are making a decision to go up there within a very, very short space of time.
"It's a question of grabbing that window whenever we can."
Seepage from the dam had increased by half in a span of eight days and DoC expected a collapse to occur in February or March. The dam's inevitable collapse from the rising lake level will release tonnes of water, causing the lahar.
The current warning level, 3a, means there is a five to 10 per cent chance of a collapse. But a rupture seems increasingly likely, with the lake filling at a rate of 10,000 cubic metres of water a day due to rain, snow and melting ice cliffs.
If lake levels rise another metre - which could be reached as soon as early next month - the probability of a lahar is 50 to 60 per cent.
While there is no blanket ban, signs at key points warn trampers of the danger, and advise them not to stop in the area.
Trampers in the area would hear the rupture, Mr Wakelin said. The 1953 lahar could be heard by people as far away as Waiouru.
Once the dam breaks, the lahar is expected to flow down the mountainside following the Whangaehu River to Tangiwai and out to sea.
Mr Wakelin said the various agencies were prepared.
"Everything's clearly being done to minimise risk to public, risk to human life."