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White Island volcano is showing signs of increased activity, GNS Science says.

The volcano, about 49 kilometres off the coast of Whakatane, is a popular tourist spot, but GNS volcanologist Michael Rosenberg says those visiting it should be cautious even though the alert level for the island had not changed.

"Eruptions can occur at any time with little or no warning. The recent changes in activity suggest that the hydrothermal system has become unstable, and as a result the risk has increased," Rosenberg said.

GNS Science has changed the volcano's code from a "normal, non eruptive state" to "experiencing signs of elevated unrest above known background levels".

The volcano's lake level quickly rose by about three to five metres sometime between Friday and Saturday last week, exposing a "vigorous" flow of gas and steam into the air, Rosenberg said.

It has risen in the past, but took much longer than the 24 hours it took to rise three to five metres on Friday and Saturday, he said.

During the past few weeks there had also been some minor volcanic tremor, including several hours on Saturday, Monday and Tuesday.

Last year and earlier this year the lake started to evaporate and exposed steam vents and two large muddy pools.

"These phenomena are typical for White Island's activity, but are the first substantial changes to occur in the last few years," Rosenberg said.

No-one lives on the privately-owned island but flights and boat-trips to the island are common.