White Island volcano's unrest is possibly being caused by magma pushing closer to the surface, GNS Science says.
GNS yesterday changed the volcano's code from a "normal, non-eruptive state" to "experiencing signs of elevated unrest above known background levels" after crater lake began to re-fill quickly and the number of tremors began to increase.
The volcano, located about 49 kilometres off the coast of Whakatane, is a popular tourist spot, but vulcanologist Michael Rosenberg said those visiting it should be cautious.
Scientists could not rule out a volcanic eruption in the future, as history showed White Island could be unpredictable, he said.
"It can erupt with virtually no warning at all and that happened in July 2000.
"It was in a slightly elevated state of unrest, higher than what it is now, and just overnight it was just bang."
However, Rosenberg also said episodes of volcanic unrest could frequently lead to nothing.
Last week, the volcano's lake level quickly rose by about three to five metres sometime between Friday and Saturday, exposing a "vigorous" flow of gas and steam into the air, he said.
"There used to be a lake there in the early 2000s, and it slowly evaporated and it was virtually down to nothing and then overnight the water level rose by maybe five metres."
It has risen in the past, but took much longer than the 24 hours, he said.
During the past few weeks there had also been some minor volcanic tremors, including several hours on Saturday, Monday and Tuesday.
"There hasn't been very much of that volcanic tremor in the last couple of years, this is one of the reasons that we issued the alert bulletin."
Rosenberg said scientists believed there was something changing in the magma underneath the volcano.
"It's either that a pulse of gas is being released from that magma and that's providing a bit of extra pressure to push everything else towards the surface, or possibly it's a small amount of magma pushing closer to the surface. We are unsure at this stage."
He said GNS Science would stop tourists from visiting the island if the volcano began to erupt a plume of ash, rocks or mud.
"From where the volcano is now, there would have to be quiet a significant change over many days to weeks before it got to the stage where we thought it would erupt," he said.
Further west, Mt Tongariro has also been showing signs of increased activity, with more than 100 minor shakes since July 11.
Rosenberg said having White Island and Mt Tongariro both on a yellow code - meaning "signs of elevated unrest" - was just a coincidence.
"Each volcano behaves in its own way. Yes, they both lie pretty much of either end of the Taupo volcano zone that runs through the North Island that's on top of the plate boundary. But that's about as closely related as they get."
What will be the main motivation for humanity's future space endeavours?Related story: (See story)
The cost of losing nature