Mt Ruapehu's risk of eruption is increasing, with its volcanic alert now at Level 2.
GNS duty volcanologist Geoff Kilgour also confirmed the mountain's crater lake temperature has risen about 20degC since mid-April.
Moderate levels of volcanic tremor continue and the lake temperature measured by the outlet data logger is 45degC, up from 25degC.
For the past couple of days it has ranged between 45degC and 46degC, he says.
"The increase in gas output, the high lake temperature and continued seismicity suggest that Ruapehu is at a higher level of volcanic unrest. As a result, GNS scientists revise the volcanic alert level to Level 2 [moderate to heightened unrest]. The Aviation Colour Code is also changed, from Green to Yellow."
Alert levels range from zero - "no volcanic unrest", to five - "major volcanic eruption".
Mr Kilgour said scientists made two visits to Ruapehu yesterday, one flight to measure the gas output and other to sample the crater lake water and make additional ground-based gas measurements.
"Volcanic gas measurements indicate an increase in the amount of both carbon dioxide (CO2) and sulphur dioxide (SO2) output ... Seismic activity at Mt Ruapehu is usually dominated by volcanic tremor. Since the volcanic earthquakes in late April the seismicity has been dominated by volcanic tremor at varying levels. The level of tremor has increased but is not exceptional in terms of the last few years."
Mr Kilgour says the new data indicated more volcanic unrest at Mt Ruapehu.
"The increased gas output coupled with high heat flow and volcanic tremor imply a higher likelihood of activity."
GNS Science volcanologists continue to closely monitor Ruapehu through the GeoNet project, he says.
The Department of Conservation has also issued a warning to climbers and trampers on Mt Ruapehu, not to enter the Summit Hazard Zone on the mountain until further notice.
The Summit Hazard Zone is the area within 2km of the centre of the crater lake. It encompasses all the peaks in the summit area, from Te Heuheu Peak at the north end of the summit area, to the upper Turoa skifield at the south.
Climbers and trekkers should refer to the Summit Hazard Zone map or use their map and GPS reading skills, to determine when they are approaching the zone.
"We recommend climbers, trampers and walkers do not enter the zone", said Paul Carr, DoC's operations manager for Tongariro.
"Guiding companies should also heed the advice and not take people into the zone.''
No ski areas, other facilities or roads on Ruapehu or elsewhere in Tongariro National Park - including the Tongariro Alpine Crossing - are affected by this warning.
