The temperature of Mt Ruapehu's crater lake is once again rising, with underground earthquakes recorded and hot water from the crater lake spilling into a nearby river.
The volcano, which is currently at Volcanic Alert Level 1, has been undergoing a number of heating and cooling cycles over the last year.
In December, the temperature of the lake rose to 43C, with around 400 megawatts of heat entering the lake. That temperature then cooled, before hitting 40C in April.
By June, the temperature had subsided again, hitting a low of just 26 degrees.
But GNS Science has today said the temperature is again rising, currently sitting at 31C, with 250 megawatts of heat flow entering the river, up from just 50 megawatts last month.
"Typically, over periods of months, Ruapehu's Crater Lake, Te Wai ā-moe, undergoes heating and cooling cycles," GNS volcanologist Craig Miller said.
"As is common during these heating phases, small volcanic earthquakes have been recorded since the lake temperature started to rise, caused by an increasing influx of hot steam into the lake."
Currently, the lake is a dark grey colour. Miller says this is typical for the volcano as sediments on the lake floor are disturbed during the influx of hot fluids and are suspended in the lake water.
The crater lake is also overflowing into the nearby Whangaehu River, which travels from the mountain through the Ruapehu settlement of Tangiwai, before entering the sea at the Whangaehu township, about ten minutes south of Whanganui.
Miller says despite the fluctuations, the volcano is stable at this stage.
"The results from our continuous monitoring of seismic activity, lake temperature and water level indicate that key monitoring parameters remain within normal ranges."
However, the level shouldn't be used to forecast future activity, Miller said.
"Mt Ruapehu is an active volcano and has the potential to erupt with little or no warning when in a state of volcanic unrest."
The volcano remains at Volcanic Alert Level 1 and aviation colour code green - one of just two volcanoes in the country above level zero, alongside Whakaari/White Island which is currently at Level 2.
The volcano last erupted in September 2007 without warning, with two lahars travelling down the mountain and a seven-minute earthquake recorded nearby.