A noticeable release of gas has been spilling out of Mt Tongariro but this did not mean an eruption had occurred, say GNS Science.
There was still a "substantial possibility" of eruption however, especially over the next few months.
Tongariro erupted for the second time this year on November 21, sending a 2km high plume of ash into the atmosphere.
GNS Vulcanologist Brad Scott said ongoing activity at Tongariro resembled a similar "volcanic episode" from the 1890s.
"After the second eruption in November we now have to consider the possibility that Tongariro might have entered an eruptive episode and this unrest could continue for several months.
"Within an episode Tongariro might quietly discharge steam most of the time, but occasionally have small eruptions with little or no warning."
Head Vulcanologist Gill Jolly said it was difficult to predict what might happen over the next few months.
"Unfortunately with active volcanoes nothing is black and white and our best assessments still have a lot of uncertainty.
"What we can say is that eruptions substantially larger than that on August 6 should give us some warning signs and at the moment we don't see any such signs ", said Dr Jolly.
GNS said they had received several reports of possible eruptions, but each incident turned out to be normal steam discharge. On a fine day, the steam plume was just more prominent.
There had been only minor seismic activity at Tongariro since 21 November. High winds meant last week no volcanic gas measurements were recorded.
GNS said gas could still be smelt downwind from Tongariro and be a minor irritant to some people.
- © Fairfax NZ News
Do you agree with increased oil exploration?