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.