A cluster of small volcanic earthquakes beneath Mount Tongariro has prompted GNS Science to raise alert levels.
Usually such quakes only occur at Tongariro at an average rate of two per year but in the past weekmore than 20 have been recorded, including some on Friday.
The earthquakes are all small in magnitude, at less than 2.5, and have been centred between the Emerald Lakes and Te Maari craters.
Volcanologist Brad Scott says GNS has raised the alert level from zero to one, which means there are signs of volcanic unrest, in a range that can reach six,
Mr Scott says they have also upgraded the aviation alert to yellow, which at this stage doesn't affect flights.
He says GNS is continuing to monitor activity and plans to deploy seismic recorders and take samples from the nearby hot springs and lakes.
He told Checkpoint the small quakes could be a sign that an eruption may happen in future.
Mt Tongariro is not a single volcano, but a complex of craters that have been active at different periods, GNS says on its website. Although often perceived as a separate volcano, Mt Ngauruhoe is Tongariro's main active vent.
In 1954 Mt Ngauruhoe produced more than six million cubic metres of lava and sporadic activity which began in January 1973 built up to highly explosive eruptions of ash in January and March 1974, and in February 1975. One of these threw a 3000-tonne block of lava 100 metres out of the crater. It has not erupted since 1977.