The volcanic alert for Whaakari/White Island has been raised to alert level two and there is a "large uncertainty" of what was driving the activity.
Changes in gas composition and continued heat flow indicated heightened unrest at the volcano, Geonet said in a statement.
Alert level 2 meant there was moderate to heightened unrest at the volcano, however, Geonet said there were no obvious signs of eruptive activity.
"While Volcanic Alert Level 2 is mostly associated with volcanic unrest hazards (including discharge of steam and hot volcanic gases, earthquakes, landslides and hydrothermal activity), potential for eruption hazards also exists and eruptions can still occur with little or no warning."
Tremor and volcano-seismicity remained low and ground deformation showed an overall pattern of subsidence, with some variability, Geonet said.
The volcano was continuing to be monitored for further changes in unrest. This was being done through a network of seismic and acoustic sensors, GNSS (GPS) receivers, cameras, and gas and visual observations.
Geonet said their monitoring equipment had degraded to the point where they could no longer provide real-time monitoring of the volcano.
"We are relying on satellite data, inconsistent GPS data, regular gas and observation flights (weather permitting) and two webcams, one of which is significantly degraded," it said.
"We are continuing to work on options to restore our monitoring ability."
Whakaari/White Island last erupted on December 9, 2019, killing 22 people.
Earlier this month, Bay of Plenty residents may have spotted large steam and gas plumes coming from Whakaari/White Island in the past few weeks.
GNS Science volcano specialist Brad Scott said the steam and gas plumes originated from several active vents on the island and were clustered in and around the craters formed in December 2019.
Scott said several factors contributed to how the steam and gas plumes appeared above the volcano.
"These include heat flow, gas output, and the presence of groundwater, lakelets or crater lakes. Humidity, dew point, air temperature and wind speed can also play a role.
"Line all of these up like we did over the last couple of weeks and you can have large spectacular steam and gas plumes over the volcano."
Scott said gas was formed from subduction and magmatic processes at depth and was discharged at volcanoes and geothermal areas.
"The gas leaks from the rising geothermal fluids and molten rock (magma). Steam-water vapour (H2O) is the most common volcanic gas constituent.
"However, there are several other gas species present. After steam, in order of abundance they are carbon dioxide (CO2) sulphur dioxide (SO2) and hydrogen sulphide (H2S). GeoNet measures the amount of these present during regular gas flights."
What is the night glow?
A night glow has also been visible since June 30.
"Our volcanic cameras have both daylight and low-light cameras installed, allowing pictures to be taken at night when there is enough ambient light (such as from a full moon).
"The low-light cameras are also able to see the hot steam and gas vents, and these appear as a "glow" on our cameras on the island.
"This usually requires moderate-strong and hot emissions from the vents on the volcano for this to occur. Observations from a flight on 15 July confirmed temperatures of 500 to 600 degrees Celsius are now present."
The glow is only visible in the near infra-red with its webcam and unlikely to be visible with the naked eye.