GNS volcanologist Brad Scott said there remained a real possibility of Tongariro blowing again.
"The volcano is in a state of unrest," he said, "I definitely wouldn't say another eruption wasn't going to happen."
Volcanologists had not been able to access the crater on foot yet - "it's just too dangerous."
Thick grey smoke, gas and ash spewed 4km into the sky from Te Maari vent on the western side of the mountain during the unexpected eruption just before 1.30pm yesterday.
It is the same place where Tongariro erupted in August, for the first time in more than a century.
Video of the eruption showed rocks of about a metre across had also been blown into the air.
Mr Scott said people up to 400km away could expect to smell sulphur from the volcano in the coming days but that was nothing to be concerned about.
The Department of Conservation (Doc) has closed its tracks within a 3km radius of the crater.
DoC spokesman Bhrent Guys said staff would remain posted at the main entrances to the national park on the central plateau tomorrow to stop curious spectators getting too close.
The world-famous Tongariro Crossing would remain closed for at least the next three days, he said.
"We hope to have the tracks open as soon as we possibly can, keeping in mind public safety."
However, the greatest risk was not the volcano at all.
"The greatest hazard anyone's going to face during this event is being run over on the road. I make that point quite seriously: two people were killed when they parked up to look at at the Ruapehu eruption in 1995."
He said there was a huge amount of interest in the eruption.
