It is one of nature's most dangerous threats, known as a lahar, and it can be far more lethal than lava flows. In Colombia in 1985, 24,000 people were buried alive when a lahar swallowed up the town of Armero, following an eruption by the Nevado del Ruiz volcano. Another lahar in 1953 caused New Zealand's worst rail disaster, killing 151 people.

Lahars are caused by a lethal combination of water and volcanic flows. Either an eruption suddenly melts huge quantities of snow and ice, sending it rushing down the mountain's flanks, or a natural dam is formed, trapping water at high altitude until the wall holding it back is ruptured. This is what has happened on Mount Ruapehu, where a huge crater lake, nearly 9,200ft up, is about to burst its banks.

The volcano, used as a setting in the Lord of the Rings films, last erupted 11 years ago, leaving a wall of tephra - ash, sandy particles and heavier material - on the crater rim. The barrier is already leaking as the lake rises, and scientists expect complete collapse within a few weeks. TV companies are vying for exclusive rights to film the spectacle.

The authorities insist there should be no danger this time.

The rise of the lake has been closely monitored over the past decade and millions of dollars have been spent on safety measures. An early warning system will be triggered by any event, causing gates to close bridges and roads in the flood's path. A special team of 'lahar cops' is on 24-hour call.

The authorities' main concern is the threat to tourism. Guests at the exclusive Chateau Tongariro resort will be evacuated if the alert goes up. "You have to realise we live and work on the flanks of a volcano, so this is second nature to us, " said the general manager, Kathy Guy.