Mt Ruapehu is experiencing an increase in lake temperature and seismic activity according to Geonet.
Duty Volcanologist Agnes Mazot says temperatures in Mt Ruapehu's Crater Lake have been on the rise since last September.
"Since 2009, we have continuously recorded the temperature of Te Wai ā-moe. Over this time, the temperature has often cycled between ~ 15 and 45°C over a period of about 12 months.
"However, in September 2018 we recorded a departure from this, and for the following 6 months the lake temperature remained elevated (at ~ 30°C).
"Over the last two weeks, the lake temperature has risen further, at a rate of around 0.5°C per day, to 42°C on April 9.
"To put this in context, only 1 per cent of post-2009 temperatures have exceeded 42°C and a similar temperature last occurred in May 2016," Mr Mazot says.
The higher temperatures have seen a rise in volcanic tremor activity over the last two weeks, which Geonet classifies as "moderate" at this time.
Mr Mazot says the mountain remains at Volcanic Alert Level 1 – minor unrest.
"In the past, eruptions at Ruapehu have occurred more often when the lake exceeded 45°C. However, a temperature of 46°C was recorded in 2016 with no eruption.
"While this is the case, it is a useful reminder that eruptions can occur with little or no warning. GNS Science continues to closely monitor Mt Ruapehu and our other active volcanoes."
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