GNS Science scientist Brad Scott said the explosion, which was followed by a small earthquake, happened quickly and with little seismic activity before the eruption.
This was despite three or four weeks of unrest at the volcano with volcanic earthquakes occurring at Tongariro and the Te Maari crater.
"If the unrest episode which we have seen for the last two or three weeks continues, we could see the volcano change from the hydrothermal towards the magmatic type of activity, but we are not seeing any indication of that at this stage," Mr Scott said.
"We have identified the unrest episode but the eruption itself gave us no warning and if further hydrothermal-driven eruptions occur, they could well be the same."
The eruption sent a plume 7000m into the clear night sky, carpeting surrounding areas in ash.
Boulders as large as 1m wide have been found strewn around the eruption site at the Te Maari crater on the mountain's northern face, and falling debris crashed through the roof of an unoccupied hut.
Scientists were closely monitoring the situation yesterday and warned the eruption could be followed by heightened activity over coming weeks.
"As with any volcano, an eruption could occur at Tongariro at any time with little or no warning and there is an elevated level of risk, particularly on the northern slopes and valleys of the mountain," Geonet said.
Dr Thomas Wilson, lecturer in hazards and disaster management at the University of Canterbury, said the activity could stop completely, continue in same-size eruptions or signal a bigger eruptive sequence, spreading thicker ash over a wider area.
"A steam-driven eruption like this could be a sign that there's magma moving into place under the volcano, and we might see a progression through to a magmatic eruption," Dr Wilson said.
"Or it could just be that the volcanic hydrothermal system has been unsettled by these earthquakes, and we're seeing an eruption as a result of this."
University of Auckland vulcanologist Dr Jan Lindsay said it was possible the unrest could continue for months.
Lake Rotoaira Rd residents began scrubbing their homes and vehicles clean yesterday, still shocked at waking to the surprise spectacle.
"I thought it was a train at first, but it was continuous and then it sounded more like a jet engine ... the whole house was vibrating," Malcolm Haitana said.
Dave Allan, whose house, farm and livestock were coated in a clay-like muck up to 6cm thick in places, described the rumbles as a "deep boom".
"I got up and looked out at the sky and it was bloody beautiful, an amazing sight, and then it went bang, and straight up ... and I thought I'd better get the hell out of there."
The eruption closed highways, delayed flights and forced the evacuation of about 24 residents of the mountainside Lake Rotoaira Rd, but the threat was downgraded by authorities as drifting rain clouds dispersed the ash.
Scientists hope to know more about the components of the ash cloud today and are carrying out a full chemical analysis.
GNS Science duty volcanologist Michael Rosenberg said the cloud would have contained tiny particles of fragmented rock, droplets of water and probably acid.
It would also have contained volcanic gases such as carbon dioxide and sulphur dioxide.
Mr Rosenberg said the ash cloud after the 1995 Ruapehu eruptions contained chemicals such as fluorine, which is toxic to animals.
"We haven't yet been able to analyse any of the ash samples that we've had passed to us, so we don't know if there are those chemicals in this ash."
