Eruptions at Mt Tongariro could continue for months, if not years, scientists say, and they are still expecting Ruapehu to let off more steam.
The eruption at Tongariro's Te Maari crater about 1.30pm on Wednesday sent a 4-kilometre plume of ash into the air and lasted about five minutes, GNS Science confirmed yesterday.
It monitored the mountain closely all day yesterday, saying activity was at a low level.
However, there remained a significant probability of a sudden eruption within the next week.
"Progressive pressure may build up over time and we think that's what's happening," GNS vulcanologist Nico Fournier said.
"At the moment we can't make any judgment calls, but the overall opinion is if there is an eruption it is unlikely to be quite big . . . the bigger the eruption the more signals you should get beforehand and we haven't seen that."
Signals included earthquakes, gases emitted and the ground swelling slightly.
"We haven't had any evidence of any of those at the moment," Mr Fournier said.
Wednesday's eruption was the second this year, after Tongariro erupted in August - the first time in more than 100 years.
Meanwhile, GNS Science vulcanologists continue to monitor Ruapehu, which it warned last week had pressure building under the crater lake and was likely to erupt.
Vulcanologist Brad Scott said personnel visited the crater lake on Wednesday to sample the water and gas.
While the results of tests taken then were not yet known, he said everything looked "quiet at the surface".
But he said there was still the likelihood that it would erupt.
The aviation colour code remained at yellow and the volcanic alert level remained at 1.
"We are monitoring Ruapehu closely, but it often does not give any immediate warning that it is going to erupt," Mr Scott said.
Since late October, small earthquakes had been occurring about 5km beneath the summit area of Ruapehu, but Mr Scott said they might not be directly related to high temperatures beneath Crater Lake.
- Waikato Times