Risk of sudden volcanic activity as Mt Tongariro, Mt Ruapehu and White Island simmer.

The steam and super-heated gases which have been pouring from the side of Mt Tongariro since its two surprise eruptions last year are set to be a feature of the volcano for years.

And nearly two months since Mt Tongariro last blew, GNS volcanologists say there's every chance of another sudden eruption, just as at neighbouring Mt Ruapehu and White Island to the north.

Fresh activity at all three volcanoes last year forced authorities to raise their alert levels, which have since been reduced.

The latest check by GNS showed steam and gas plumes at the Te Maari craters on Mt Tongariro's northern face - the site of eruptions in August and November - had become a continuous feature at the site.


The gas was coming from a large fumarole and crack in a cliff just east of the Upper Te Maari crater, while the main Upper Te Maari crater was also discharging gas but at a lesser rate.

Volcanic gas from the plume, which could still be smelled downwind of the volcano, was stronger on some days because of atmospheric conditions.

Volcanologists expected these plumes to linger for several years.

The puffs of steam had occasionally led to excited reports of eruptions, GNS volcanologist Dr Craig Miller said.

A section of the Tongariro Alpine Crossing, between Emerald Lakes and Ketetahi Rd, remained closed and Dr Miller said trampers should stay cautious when on the mountain.

"We still expect there could be a November type eruption."

The Department of Conservation reported large numbers of visitors to Tongariro National Park this summer, with vehicle congestion forcing the temporary closure of Mangatepopo Rd.

Adrift NZ tour guide Stewart Barclay, who chairs a group of 30 users of Mt Tongariro, said business was up because of the volcanic activity and would likely increase further when the crossing was fully open.

Meanwhile, the alert level at White Island was lowered from two to one this month, reflecting the end to an eruptive cycle that began in August, more than a decade after its last eruption.

Scientists visiting the island on New Year's Day found a small lava dome that had risen throughout the cycle had not grown since previous inspections the month before.

Temperatures measured at the dome and at a nearby hot lake, which had reached 200C to 240C and 70C to 80C respectively, had also not changed. It suggested magma was not pushing out the cooler plug blocking it, Dr Craig Miller said.

"It's a bit like an old tube of toothpaste with a dry cap on top. If you stop squeezing it stops coming out.

"But we think there's still probably a lot of magma at a shallow depth - and while it remains there, there's always the potential for things to happen."

A similar process was likely occurring at Mt Ruapehu, where the crater lake was remaining worryingly docile despite fiery temperatures just below.

Since March last year, lake temperatures had hovered between 20C and 25C while temperatures beneath ranged between 700C and 800C.

The lake's temperature had reached 40C in previous years, and its stability suggested gases were being blocked, bringing the potential of a sudden and violent blow.

A 2km exclusion zone around the summit remained in place.

"Mt Ruapehu is still in a state of heightened unrest and people should be aware that it can do things with very little warning," Dr Miller said.

"It's got the ability to throw rocks large distances and send lahars down the mountain."

Volcano watch
White Island
This week downgraded from alert level two to one (out of five). A recently-established lava dome has stopped growing, but scientists say a column of magma not far beneath still poses a threat.

Mt Ruapehu
Remains at alert level one with an exclusion zone around the summit. A suspected blockage below the crater lake may be causing a gas build up that could result in sudden eruption.

Mt Tongariro
Remains at alert level one with part of the Tongariro Alpine Crossing closed. Emitting large amounts of gas and sulphur dioxide and scientists still expect a repeat of the last eruption in November.