A series of quakes beneath Mt Tongariro have resulted in the alert level for the volcano being raised.
GeoNet said typically Tongariro experienced two such quakes - magnitude 2.5 or less - a year but there had been more than 20 since July 13.
It said the sequence started on July 13 and declined for a period, before restarting on July 18 and increasing in number yesterday.
''These indicate unrest at Tongariro and give reason to change the Volcanic Alert Level to level 1 [from 0] and the aviation colour code to yellow [from green].''
''These earthquakes are small (magnitudes <2.5) and have only been well recorded by a few of the seismometers in our permanent network. The earthquakes cluster in a zone between Emerald Crater and the Te Māri craters at 2-7km depth.
GeoNet said to better understand the significance of the quakes it was planning to deploy portable seismic recorders around the epicentres and conduct sampling of selected hot springs, crater lakes and fumaroles in the area.
Mt Tongariro is a volcanic complex that lies to the north of Ngauruhoe. It consists of numerous craters and vents.
The Te Mari craters lie about two kilometres east of Ketetahi hot springs on the north side of Mt Tongariro. The Te Mari craters are the last craters to be active on Tongariro. Ash eruptions have been recorded from Tongariro from 1855 to 1897, as well as unconfirmed activity in 1926-27.
- © Fairfax NZ News