Ngauruhoe rumbles with activity 

Scientists in the central North Island are keeping a close eye on Mount Ngauruhoe after an increasing number of earthquakes.

The volcano has been largely silent since it last blew in 1975.

But for the past two weeks, Ngauruhoe, which is located in a chain of volcanoes that sit in Tongariro National Park, has been showing signs of activity.

"These are the initial signs that something is happening and it may just be that these earthquakes just die away and that'll be it. Or they may increase," says GNS volcano geophysicist Craig miller.

This does not necessarily mean an eruption is imminent - the mountain is at alert level one on a scale of up to five - but scientists are regularly checking seismic stations around the mountain.

Ngauruhoe's neighbour Mount Ruapehu is also being monitored.

Scientists say a build-up of debris in its crater lake could cause a lahar (mud flow) as early as next summer.

"A lahar is just a volcanic mudflow basically, water flowing in this case out of the crater lake down the Whangaehu River which is the outlet for the lake, and as it does it'll be picking up mud debris boulders anything else that's in its path," says DOC community relations officer Dave Wakelin.

The last major lahar on record was that which caused the Tangiwai disaster in 1953 after it took out the rail bridge at Tangiwai, killing 151 train passengers.

There are currently early detection systems in place, scientific monitoring and emergency response plans to reduce the effects of any potential future risks.