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A surge of vigorous hydrothermal activity at White Island has vulcanologists warning that even greater volcanic unrest could be on its way.
GNS Science has stepped up monitoring of the Bay of Plenty island after an increase in activity since late last year.
The island's volcanic alert remains at level one of five, indicating signs of unrest, while its aviation colour code remains at yellow, which means there are signs of elevated unrest.
GNS vulcanologist Brad Scott, who visited the island yesterday, said the hydrothermal activity was "some of the most vigorous I have seen at White Island for many years".
"This type of activity usually leads to stronger volcanic activity and is a significant concern."
Future eruptions were possible with little or no warning.
Mr Scott's observations confirmed that hydrothermal activity in the small "hot lake" had increased.
Steam and gas was "doming up" the surface of the lake, bringing large amounts of sediment to the surface, often with vivid white steam and gas flashing around the base.
Mr Scott said even stronger events happened every so often.
A lava dome that was first observed in late November had not changed since it was last observed at the start of the year.
Activity at the island has been increasing since late 2012 and was now semi-continuous.
GNS Science said an eruption like one in August last year could occur with little or no warning, despite the alert level remaining at one.
The increased level of unrest did not mean there was a hazard to visitors, but GNS said caution should be exercised when visiting the island.