Mt Tongariro is one of three major volcanos in the central North Island, sitting alongside mounts Ruapehu and Ngauruhoe.
The mountain has been dormant for 115 years - the last series of eruptions began in November 1896 and lasted until October 1897.
According to GeoNet, Tongariro is a complex of multiple volcanic cones constructed over a period of 275,000 years.
The mountain's active vents include Te Maari, Emerald, North Crater and Red Crater.
Seismic activity indicating unrest was last detected at Mt Tongariro during 2001.
Prior to that, there were five reported eruptions from the Te Maari craters between 1855 and 1897.
A large eruption in 1869 was accompanied by an earthquake and formed the upper Te Maari Crater, named after a Maori chieftainess.
Maori descriptions of the event spoke of "bright red flame through the smoke that would burst and fall like snow".
The mountain again let loose in November 1892, spewing steam, mud and boulders 2000-3000 feet into the air.
A journalist for the Auckland Star reported a "heavy booming" from Tongariro on February 9, 1892.
"I had a glimpse of the mountain for a few minutes to-day; it was sending up steam and smoke to a tremendous height."
The 1897 eruption dumped 50 millimetres of ash on the Desert Road and ash drifted as far as Napier.
GeoNet says Tongariro typically experiences two quakes of a magnitude 2.5 or less each year but there had been more than 20 since July 13.
There are six alert levels of volcanic action, increasing in seriousness from zero to five. Alert level one indicates "signs of volcano unrest".
For the alert level to be lifted to two - "minor eruptive activity" - there would need to be an eruption.
- HERALD ONLINE