KEY POINTS:
Mt Ngauruhoe - Mt Ruapehu's volcanic neighbour - is getting restless.
The two mountains are believed to share the same magma source beneath the Earth's crust.
Mt Ngauruhoe has shown "significant" increases in earthquakes over the past eight months, stirring after more than 30 years of deep sleep.
The unrest at both mountains relates to a special low-frequency type of earthquake, which is often linked to the underground movement of gas, hot water or magma.
However, GNS volcanologist Graham Leonard said while the two volcanic systems might share the same magma source they were "quite divorced" from one another.
The events needed to be kept in context, he said.
Before Sunday's lahar between four and 60 small earthquakes were recorded daily at Mt Ruapehu's Crater Lake. But in the 24 hours following the lahar about 100 earthquakes were detected, Dr Leonard said.
"It's nothing we would call a crisis, but there is a slightly elevated risk of a geothermal eruption. You could visualise it as a geyser of about 40m to 50m above the surface of the lake."
The increased unrest had almost certainly been caused by the crater lake emptying out 1.29 million cubic metres of water, causing its level to drop 6m.
He could not predict the likelihood of an eruption. "The probability is very difficult to calculate," he said. "There aren't enough samples or historical data to go on."
However it was important people in the upper Whangaehu valley and in the Crater Lake area knew of the increased danger.
"It could spill a little bit of water into the catchment ... [but] there's a low probability of it occurring at all. If it occurred the impact would be minor.
"There's a hint they've tapered off now but because things are so variable we need to watch this over the course of a week."
Sensitive equipment was picking up the tremors, which measured less than 1 on the Richter scale, Dr Leonard said.
People could not feel them, even if standing at the crater rim.
The earthquakes at Ngauruhoe are of a similar size to those at Ruapehu.
According to the GeoNet monitoring project, Ngauruhoe had until recently been shaking at a rate equal to twice daily between 1975 and May last year. However, since then it has regularly had between 20 and 30 earthquakes a day, peaking at more than 50 shakes within 24 hours.
