A "small batch of magma" could explain the ash seen rising more than half a kilometre above Whaakari/White Island over the weekend.
Ash emissions and a strong sulphur dioxide were detected at the offshore volcano on Sunday but because of the lack of real-time data, GNS Science said in astatement it could not definitively say whether a small eruption had occurred.
Web cameras showed minor ash emissions from an active vent behind the crater lake while a "strong" sulphur dioxide gas signal was detected by satellite.
The ash was light brown in colour and rose about 600m above the volcano, but was not observed beyond the island. A steam plume could be seen from the Bay of Plenty coast.
GNS Volcanologist Steven Sherburn said the movement of magma was the most likely explanation for the ash emission.
View from North Rim camera showing ash emission from an active vent (brown plume) on Sunday. Photo / GNS Science
"We believe the most likely explanation for the ash emission and sulphur dioxide plume visible on Sunday was a small batch of magma moving into the shallow part of the volcano.
"One of our seismic recording sites on Whakaari/White Island started working again [on Monday] morning, however, the lack of real-time seismic and acoustic data on Sunday prevents us from being definitive about a small eruption having occurred at that time," Sherburn said.
Last month, GNS reported its near real-time monitoring on the island had been lost after instrument failure since the 2019 eruption.
Sherburn said limited data over the past few weeks caused uncertainty about what was driving the activity.
"We are uncertain why our seismic station started to work again, and do not know if it will keep working in the coming days," he said.
The Volcanic Alert Level on the island remains at 2.
Sherburn said GNS would be increasing the frequency of its gas and observation flights to the island until they could service the on-island equipment and power supplies.
