Mt Tongariro has been quiet overnight, with no reports of further seismic activity as scientists try to figure out whether it is likely to blow again.
The weather this morning was clear around the mountain, which erupted shortly before midnight on Monday, spewing ash and rock high into the air.
GNS Science volcanologist Steve Sherburn said that apart from than a minor earthquake, the mountain was quiet overnight.
Massey University scientists yesterday collected ash samples which will be analysed to try to determine what sort of eruption Monday's was.
"One of the key things the ash samples will tell us is whether it is a phreatic, steam-driven eruption, or a phreatomagmatic eruption, with magma driving it from the bottom," Mr Sherburn said.
If it was the latter, it would be more concerning because there was a greater chance of further large eruptions.
"One of the key things we always want to find out in situations like this is what kind of eruption it was. We want to be able to work out as well as we can what may happen in the future and plan accordingly."
Results from the samples would likely be ready this afternoon.
Mr Sherburn said the priority today was to observe the volcano from the air but that would depend on the conditions.
Attempts to fly over Tongariro yesterday were hampered by low cloud, and while it was clear this morning, Mr Sherburn said the weather was forecast to deteriorate later today.
An observation flight would help scientists pinpoint exactly where the eruption came from. It was possible there were many as three vents, Mr Sherburn said.
"One of the things we have to draw is a hazard map so people that are working or visiting on or near the volcano can know what are the key areas that hazards may occur in.
"One of the hazards is what we call ballistics, basically big rocks. We usually define a roughly 3km radius around the crater for the typical distance these things can fly. That's obviously a really high hazard, because if one hits you and it is more than about a pea-size, it will probably kill you."
Mr Sherburn said what the earthquake would do next was anyone's guess.
"This is the question that we simply do not have an answer for, particularly when volcanoes are in a period of waking up they can have an eruption and then do nothing for days and weeks and then gradually build up over time, or it could be something like the Ruapehu 2007 eruption or Raoul Island 2005 eruption which were just one-shot wonders."
