A lahar could occur on Mt Ruapehu as early as next month, with the crater lake expected to reach capacity in as little as two weeks.
Department of Conservation scientist Harry Keys said a lahar, or volcanic mud flow, was inevitable.
The crater lake was 97 per cent full and
expected to reach 100 per cent in two to six weeks.
Mt Ruapehu's 1995-96 eruptions left several metres of rock fragments and particles around the crater rim. At the lake outlet above the Whangaehu Valley, the ash is about 7.5m high.
As the lake water seeps through the debris the likelihood of a lahar increases and the higher the water seeps, the larger it will be.
About 60 lahars have come down the side of Ruapehu in the past 150 years.
An embankment, or bund, has been built lower on the mountain to try to prevent a lahar spilling into the Waikato Stream and into Turangi.
Dr Keys said the chances of the bund breaching were less than 1 per cent but if it did the lahar would reach Turangi in four to eight hours.
Local and regional councils, police and other emergency services are working to prepare for the event. The road bridge at Tangiwai is being lifted 2m, high enough to avoid lahar damage.
The nearby Tangiwai memorial will be removed and put into storage until after the lahar, when a safer location will be found.
A sophisticated early warning system is in place and DoC estimates the lahar will take 90 to 120 minutes to reach the Tangiwai road and rail bridges.
It will then take 22 hours to make its way to the sea near Wanganui.
Police will be the first to respond and will block State Highway 1 and State Highway 49.
Eight police are being relocated to Waiouru on January 5, to be on standby.
- NZPA
