Volcanologists say a steam plume rose from Mt Ruapehu on Tuesday morning up to 1.5 kilometres in the air.
GNS Science said the cloud was visible for 45 minutes, from 9.30 am, but "no seismic or acoustic activity accompanied the plume appearance".
This suggests it "was not generated by strong activity in the lake".
There has been elevated volcanic unrest on the mountain for the last six weeks and in the last week it has "exhibited the strongest volcanic tremor in two decades".
The crater lake temperature has risen to 39C this week, and currently, nobody is allowed within a 2-kilometre radius of the summit. The Whakapapa and Tūroa ski fields are on the Department of Conservation land and are further down the slopes.
Tongariro National Park safety ranger Theo Chapman told RNZ that at Volcanic Alert Level 2, some lifts and trails might be closed, but not the whole mountain.
"We all anticipate there being a ski season," he said.
"Should an eruption occur then we look at some reflex closures ... But even then it doesn't mean it's going to stay closed. The situation is reasonably fluid."
Ruapehu last erupted in 2007.
Ash, rocks and water spread across the summit, producing lahars in two valleys - one in the Whakapapa skifield.
If this happens again, during ski season, people will be told to move out of the valley, and take shelter from rocks and ash clouds behind banks and ridges.
"If an eruption is detected then there are audible sirens and a voice message that are activated in high-risk lahar paths," Chapman said.
Some scientists are optimistic the mountain can stay open at Volcanic Alert Level 2.
Massey University disaster management professor David Johnston told RNZ: "I think the safety is being well-managed"
He planned to ski at Ruapehu this winter.
"It will be open if it's safe to do so, with hopefully good snow, good skiing. But we cannot predict the future, it's like trying to forecast the weather forecasts, or predict the weather six months from now, three months from now."
Dr Harry Keys has bought a season pass this year.
He spent decades helping develop volcanic risk management plans in the Central Plateau and said one of the biggest safety challenges was education.
Dr Keys told RNZ that some people did not look at the posters on the field or hazard maps online.
"Around about 80 to 90 percent of people get it but that means 10 to 20 percent of people don't get it, and that's a bit distressing."
He said there had been eight dangerous near-misses at Ruapehu since the 1940s and safety plans had been improved each time - especially after the September 1995 eruption just after fields closed for the day.
"People were quite high in the mountain still, there was no one in the T-bar area where the lahar came down but if they had been, they would have been in danger."
Ruapehu Alpine Limited, which runs the ski fields and Sky Waka gondola, declined requests for interviews.
In a Facebook post three weeks ago it discouraged people from doing the Skyline Walk near the crater, but it said only one chairlift, Tūroa's High Noon Express, was in the 2-kilometre crater exclusion zone.
Documents released to RNZ under the Official Information Act show the business reviewed its safety plans in response to the Whakaari eruption and released more detailed maps in 2020 showing predicted lahar paths and evacuation areas.
Ruapehu Mountain Clubs Association president Linda Danen encouraged skiers not to muck around if sirens go off.
"You won't ski ahead of a lahar, you've got to get to higher ground and that might mean taking your skis off and just going, just walking uphill."
GNS scientists said last night it was unlikely there would be an eruption in the next month.
Its online bulletin said a large eruption, as seen in the mid-1990s, was "very unlikely" and would probably follow a sequence of smaller eruptions.
Ski season normally opens at Ruapehu in June and July, with thousands of people on the slopes at once, on busy days.
RNZ
