Mt Ruapehu has been moved to volcanic alert level 2 following rising temperatures at its crater lake.
The lake undergoes heating and cooling cycles over a period of months. A new heating cycle has started with temperatures reaching 31C.
The rise in temperature is accompanied by strong levels of volcanic tremor.
The last time Mt Ruapehu was moved to volcanic level 2 was in December 2020.
GNS Science volcanologist Craig Miller said there was an increased likelihood of eruptive activity as strong tremor is indicating increased gas flux through the system.
"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."
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.
Miller said the lake was currently overflowing into the Whangaehu River, but the flow level was believed to be minor.
"The lake colour is expected to change to a darker grey colour as sediments on the lake floor are disturbed during the influx of hot fluids heating the lake."
The aviation colour code remains at green.