A series of small earthquakes has caused Mt Tongariro's volcanic alert level to be raised.
GNS Science says more than 20 small volcanic earthquakes - all below 2.5 - have been recorded beneath Mt Tongariro.
The sequence started on July 13, but soon declined. It restarted on Wednesday and the number of quakes increased on Thursday and Friday.
The unrest caused GNS Science to raise the volcano alert level from 0 to 1.
Volcano alert levels range from 0 to 5. Alert level 1 indicates there are signs of volcanic activity.
Alert level 5 would be in place in the event of a large, hazardous eruption.
Historic seismic data shows small volcanic earthquakes are common at Mt Tongariro, but they usually only occur at a rate of two per year.
Seismic activity indicating unrest was last detected at Mt Tongariro in 2001.
Volcanologists are continuing to monitor the earthquake activity.
GNS Science will be deploying seismic recorders around the epicentres of the earthquakes and sampling selected hot springs, crater lakes and fumaroles in the area.
Ash eruptions have been recorded at Mt Tongariro from 1855 to 1897, as well as unconfirmed activity in 1926 and 1927.