Strong tremors have been detected at Mt Ruapehu, says Geonet.
Mount Ruapehu Photo: 123RF
The volcanic alert level at Mt Ruapehu has been raised to two. It was previously at one.
In a statement, duty volcanologist Craig Miller said another heating phase had started at Mt Ruapehu and Crater Lake (Te Wai ā-moe), with the temperature now reaching 31C.
It was currently overflowing into Whangaehu river.
"Typically, over periods of months, Ruapehu's crater lake undergoes heating and cooling cycles. A new heating cycle has started and has been accompanied by strong levels of volcanic tremor (volcanic earthquakes)," he said.
"We consider there is an increased likelihood of eruptive activity as strong tremor is indicating increased gas flux through the system.
The Department of Conservation has closed an area 2km around the crater lake. Photo: Department of Conservation (Tongariro)
"Despite an increase in gas flow, the lake temperature is only responding slowly, suggesting a partial blockage may exist in the vent beneath the lake. This could allow pressure to build up within the volcano."
The Department of Conservation (Tongariro) said the area 2km from the centre of Te Wai ā-moe/Crater Lake was closed to the public and concessionaires due to the increased risk of eruption.
It said eruptions could happen at any time and the closer someone was to the lake, and the longer they stayed, the greater the risk.
Miller told RNZ the pressure building in Mt Ruapehu could lead to an eruption, but could also pass without incident.
He said the alert level rose because of the tremors, which started in the middle of last week, and because the volcano was not responding as expected to gas thought to be flowing through.
"There's something getting in the way in the middle that's blocking this gas flow and that's what we're concerned about, that it could be building up a pressurisation which, if that failed catastrophically, it could lead to an eruption.
"But it could equally dissipate as well."
That could happen if the gas pulsating through the volcano turned off.
Tremors were typically caused by the flow of gas into the shallower part of the volcano, which affected the lake temperature.
Like when someone stood on a garden hose, there was lots of gas trying to get into the lake, but only a small amount making it, Miller said.
"We're just concerned that there could be some pressure building up in the shallower part of the volcano. That's because the gas isn't really flowing as freely as we would like into the bottom of the lake.
"That's really what's prompted us to get the alert level up."
Miller encouraged people planning to visit Mt Ruapehu and the surrounding area to check the latest advice from the Department of Conservation.
The lake temperature peaked at 32C in January and since then, through to late February, it declined to about 27-28C.
The heating trend became established from 13 March.
"Volcanic alert level 2 indicates the primary hazards are those expected during volcanic unrest; steam discharge, volcanic gas, earthquakes, landslides and hydrothermal activity. While volcano alert level 2 is mostly associated with volcanic unrest hazards, eruptions can still occur with little or no warning."
Mt Ruapehu had a series of eruptions over a 13-month period in 1995/1996.