Mt Ruapehu is being closely monitored for signs of activity after the active volcano's crater lake (Te Wai ā-moe) heated to 43C, prompting an escalation in alert levels.
The active stratovolcano, 23km northeast of Ohakune and 23km southwest of Lake Taupo, last erupted in 2007.
The heating, which GeoNet says has been accompanied by "bursts of volcanic tremor and a marked increase in the amount of gas", has prompted the government agency to increase the volcanic alert from level one to two.
GeoNet volcanologist Mike Rosenberg says the amount of carbon dioxide and sulphur in the Central Plateau volcano's gas output is now at its highest point in more than 20 years, while the energy input into Te Wai ā-moe has doubled in the past month alone.
The flow of gases and hydrothermal fluids through the lake shows the underlying vent area is open, he says.
"Mt Ruapehu is an active volcano and has the potential to erupt with little or no warning when in a state of volcanic unrest," Rosenberg explained.
"Since 2007, crater lake temperature has exceeded 40C a number of times, without leading to an eruption. However the combination of the increased lake temperature, volcanic tremor and gas output have motivated the alert level change."
Volcanic alert level 2 is not a forecast of future volcanic activity, but indicates that the primary hazards of being nearby are steam discharge, volcanic gas, earthquakes, landslides and hydrothermal activity.
"[However] while volcano alert level 2 is mostly associated with environmental hazards, eruptions can still occur with little or no warning," Rosenberg cautions.
In addition to the volcanic alert level increase, the aviation colour code has also increased from green to yellow, indicating "signs of elevated unrest above known background levels".
GeoNet and the National Geohazards Monitoring Centre will be monitoring Mt Ruapehu's volcanic activity closely over the coming days and weeks, using a combination of sensors, GPS receivers, visits to Te Wai ā-moe and gas flights.