It has now been confirmed that the White Island volcano erupted; the first time since 2001.

White Island tended to have volcanic episodes which lasted a few months to a few years, so this could just be the start of more to come, GNS vulcanologist Michael Rosenberg said.

Scientists were scrambling to figure out yesterday whether the volcano, off the coast of Whakatane, had erupted earlier this week, but a visit to the island this morning confirmed it had.

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An ash cloud of about 200-300 metres suggested it was a weak eruption, GNS vulcanologist Brad Scott said.

"This is the first ash emission from White Island since February 2001 and represents the start of a new phase of volcanic activity at White Island," Scott said.

"Visitors to White Island are now at the highest level of risk since the end of the 2001 eruptions.

‘‘Additional hazards to visitors to the island now include the health effects of volcanic ash and acid gas exposure, including respiratory issues, skin and eye sensitivity to acid gases."

Black ash was still charging out of a vent on the south-west corner of the crater, while new craters had been formed from material ejected from the explosion, Scott said.

The island showed volcanic activity at 11.30pm on Tuesday, less than 24 hours after Mt Tongariro erupted for the first time since 1897.

Although the two volcanoes are on the same tectonic plate, the eruptions were not connected, Rosenberg said.

While Mt Tongariro continued to emit steam and gas, White Island was now only emitting ash.

Ashfall from White Island is isolated to the immediate area, which is 48km off the Bay of Plenty coast.

The volcano is New Zealand's most active and largest cone, but only a quarter of it is visible as the rest is under sea.

It is privately-owned but is a hot tourist spot, with many sight-seeing tours by either boat or plane.

The last big eruption, in 2000, saw a new vent develop, displaced the main crater lake and formed a new 150 metre-wide crater.

It followed a series of eruption episodes, which first started in 1998 and ended in February, 2001, Rosenberg said.

MT TONGARIRO

Early testing of the ash from Mount Tongariro showed moderate levels of the potentially toxic chemical fluorine.

The volcano's crater spewed rocks and ash when it came to life for the first time in more than 100 years at 11.50pm on Monday night.

GNS Science sent samples of the ash to Massey University for analysis, and the preliminary results have been released.

Professor Shane Cronin said the tests showed moderate levels of soluble fluorine, similar to ash from Mt Ruapehu in 1995 and 1996.

More than 2000 animals died of starvation and exposure to fluorine during the Ruapehu eruption as grass was contaminated by ash.

But Cronin said due to the restricted distribution and very thin ash fall this week, there was no current health or agricultural threat beyond the immediate vicinity of the volcano.

"In addition, heavy rainfall since the eruption has removed much of the ash and associated contaminants."

If future, larger eruptions were to occur with similar concentrations of fluorine, a significant agricultural hazard could be anticipated, he said.

"Ashfall can impact pastoral grazing systems by covering pasture, meaning that it is ingested by grazing sheep, cattle and deer."

If significant ash was ingested by livestock there could be a risk of the disease fluorosis. Deer were likely to be the most susceptible to the disease, followed by cattle with sheep being the most resistant.

Some livestock could also be put off grazing which could lead to starvation, Cronin said. It could take four to 10 days for animals to die if no other food was available.

"Experience from the 1995-96 eruptions has shown that ash coverings greater than two millimetres, low-grazed pastures, and low rainfall following ash deposition are critical factors increasing hazard," Cronin said.

"Further, livestock drinking water in open troughs may be contaminated. Additionally, rural dwellers with roof-catchment drinking water sources should be vigilant in avoiding ash runoff into water tanks.

"During and immediately following ashfall the intake pipe to water tanks should be disconnected until ash has washed off the roof with rain."

Canterbury University's Tom Wilson, from the natural hazard research centre, said the latest eruption was "a nice reminder that there is a possibility that this could be hazardous if there is a future eruption of the volcano.

"It's something we should perhaps just be prepared for,'' he said.

GNS Science volcanologist Michael Rosenberg said the tests also showed there was very little or no new magma in the ash.

"This suggests that the eruption was predominantly steam driven, but the involvement of magma in the future cannot be ruled out."

Rosenberg said Mt Tongariro had been quiet overnight, though there were still a few small earthquakes.

Scientists were currently observing active vents and new deposits from a helicopter.

"We also hope to measure gas output from the volcano using airborne platform and by road, weather permitting," he said.

Meanwhile, Rosenberg said volcanic tremors at White Island were at low levels, after ash was ejected yesterday.

A tremor was recorded at about 11.30pm on Tuesday and was followed by another short burst of activity about 3am Wednesday.

- © Fairfax NZ News