Volcanic activity at White Island is the most "vigorous" in years and could mean a large explosive eruption is imminent, GNS Science says.
The volcano, off the Bay of Plenty coast, is the country's most active and showed heightened signs of unrest last year.
But it is the increased hydrothermal activity in the last three weeks which is of "significant concern", GNS vulcanologist Brad Scott says.
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"I don't think I've seen anything this strong since the late 90s and 2000s."
The activity then led to a large eruption in July, 2000.
"The hydrothermal activity is 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," Scott said.
"Increases in hydrothermal activity often leads to explosive eruptions and that can be quite dangerous for visitors."
The activity may mean an eruption is imminent, but there is also a chance it could cease.
The volcano poses no threat to the mainland, however, as it is 48 kilometres from shore.
But White Island is a popular tourist attraction, with boat and flight tours heading there daily.
If it does explode, ash may reach the mainland and Bay of Plenty locals might smell gas, Scott says.
Scott's observations were based on a visit to the island yesterday.
Hydrothermal activity in the small 'hot lake' had increased, large amounts of sediment had surfaced and vivid white steam and gas was flashing from the base.
This was the sort of activity that had been increasing since late 2012 and was now semi-continuous, Scott says.
The lava dome that was first observed in late November has not changed since earlier this month.
There were elevated levels of volcanic tremors, likely generated by the hydrothermal activity, Scott says.
As usual, the volcano could erupt with little or no warning.