The heightened volcanic alert level on Mt Ruapehu has been described as a "kick in the guts" for one tourism business.
GeoNet yesterday raised the volcano alert to level two, indicating moderate volcanic unrest.
A 2km exclusion zone was established around the mountain's peak, which means the crater lake, Te Wai ā-moe, is no longer accessible for guided walks.
Stewart Barclay, director of Adrift Tongariro Guided Adventures, took a group up to the crater lake just two days ago.
"The lake was looking pretty grey when we were up there," he said. "I remember what [volcanologist] Dr Harry Keys, who I talk to quite a bit, was saying, when it's grey, it means the vents are opening up a wee bit.
"I was up again on Wednesday prior, and it was pretty grey then. It's generally quite a lovely milky blue.
He'd noticed some other activity a bit earlier in the season.
"Not so many weeks ago there was quite a good steam puff that we videoed as it kind of came off the lake and then went westwards.
"We kind of thought that was definitely a minor increase in activity. So we made sure we turned around and got out of there quick smart."
The volcanic alert bulletin for Ruapehu was issued just after midday yesterday, as the temperature of Te Wai ā-moe hit 43C.
Such heights aren't uncommon, but GeoNet said what caused the escalation of the alert level were the accompanying bursts of volcanic tremor and the marked increase in the amount of gas passing through the lake.
Adrift Tongariro was called yesterday morning and given the news. Barclay said it was hard to take, after a year which has hit tourism operators particularly hard anyway.
"So our summer's down to about 10 per cent of what it was, and this is just another little kick in the guts.
"We've taken the attitude that for the next 12 months we'll just ride whatever gets chucked our way, and this is just another little stone thrown at us."
The High Noon Express ski lift - part of Ruapehu Alpine Lifts' Turoa skifield - is within the 2km no-go zone.
Chief operating officer Travis Donoghue said that particular ski lift wasn't open during the summer months and so their normal service was carrying on.
"The Sky Waka gondola for us, and subsequent sightseeing, at this stage is not affected.
"What we do though, of course, is continue to respond to any recommendations and protocols from any agency in respect to any activity at Mt Ruapehu.
"If there was a further escalation of alert levels, that could potentially have an impact on operations.
"But again, we work with the relevant agencies advice around any changes and the timely communications on those means everything's achieved in a timely way."
Volcanologist at the University of Auckland, Professor Shane Cronin, told Morning Report the 2km buffer zone is a great idea.
"We get a bit of a false sense of security with this volcano so Te Wai ā-moe looks peaceful and calm, the top part of Ruapehu is covered by the deep lake and so if that lake wasn't there, what we would see would be something like Whakaari, it would be lots of steaming ... some liquid suflur, we would see lots of boiling and so on."
Cronin said he's concerned going from level 2 to level 3 requires an eruption and there needs to be another step in the system to indicate an eruption is more imminent.
The last time Mt Ruapehu's alert was raised to level 2 was four years ago and it stayed there for four months.
When Whakaari / White Island erupted last year, it was also at level 2.
Ruapehu District mayor Don Cameron said everyone was aware of what could happen.
"I wouldn't say it's a level of concern yet, but we are at a heightened level of making sure that everybody is aware there could be a further heightening of unrest, and that people need to be aware of it, so that, if for instance, there are people up on the mountain, they can get them down and out really quickly and safely."
Meanwhile, the Department of Conservation will be conducting an aerial sweep over the maunga on Tuesday morning.