Mount Ruapehu's Crater Lake (Te Wai ā-Moe) has been heating in response to volcanic earthquakes beneath the active volcano.
GeoNet confirmed the lake's temperature has risen from 24C to 40C due to hot gases and hydrothermal fluids following the earthquakes, which were recorded in February.
Volcanologists have collected water samples this week to understand the volcano's current activity, GNS Science duty volcanologist Art Jolly confirmed in a statement on Wednesday. The chemistry results are expected by next week.
"Volcanic tremor increased in response to the volcanic earthquakes, peaking in early March, then declined slowly until today," Jolly said.
Within three weeks in February, the amounts of carbon dioxide (CO2) and sulphur gases (SO2 and H2S) in the atmosphere above Mt Ruapehu increased - but Jolly says the recent values are not unusual.
Grey sediment and sulphur slicks have previously been observed on Crater Lake's surface, which has since changed to a uniform grey colour.
The current level of volcanic activity remains at Volcanic Alert Level 1. The aviation colour code remains green.
"Mount Ruapehu is an active volcano and has the potential to erupt with little or no warning when in a state of volcanic unrest," Jolly reiterated.
"GNS Science and the National Geohazards Monitoring Centre continues to closely monitor Mt Ruapehu for further signs of activity."
Although Mount Ruapehu last erupted in 2007, there have been a number of GeoNet alerts over the last 13 years warning of irregular activity. Its last major eruptions were in 1995 and 1996.