The Mt Tongariro eruption was probably caused by superheated water being forced out of the volcano's Te Maari vent, spewing existing rock into the air, scientists say.
Dr Thomas Wilson, lecturer in hazards and disaster management at the University of Canterbury, said the volcanic activity most likely occurred because heat from Tongariro's magma chambers turned water into steam.
That, combined with volcanic earthquakes and seismic activity in the area - which two weeks ago caused GNS to raise the volcanic alert to Level 1 - caused the eruption.
"There was an explosive venting of water vapour and other material. Rock that was in place around that vent was ejected and fragmented as part of that eruption and that's why we've got ash falling across the area."
Associate Professor Phil Shane, from the School of Environment at the University of Auckland, said it was a "grey area" whether the eruption of Mt Tongariro was connected to recent activity at White Island.
There was a small eruption at the Bay of Plenty volcano on Sunday and last Saturday the water level in the Crater Lake rose by about 3m to 5m.
"The volcanoes are too far apart for them to be directly linked, but then they are linked because all the volcanoes from Tongariro through the central North Island and even out to the Kermadec Islands are all part of the same subduction system - one plate sitting above another," Dr Shane said.
But given that the two volcanoes were hundreds of kilometres apart, Dr Shane said it was "just fortuitous" they both showed activity at the same time.
He said it could be up to a week before scientists knew exactly what triggered the Mt Tongariro eruption.
A GNS Science volcanologist told reporters the eruption had "just crept up" on them.
Dr Wilson, who travelled from Christchurch to National Park yesterday, said eruptions such as Monday night's, while a bit of a surprise, should be expected and were normal for seismic areas such as Mt Tongariro.
He said people living in Auckland, which has 49 volcanoes, should not worry that there could be a surprise eruption because it was very quiet seismically with very few earthquakes beneath the city. "So if we saw anything underneath Auckland, we'd be very, very attentive."