Mt Ruapehu saw "elevated" volcanic tremors in December of 2022, with its crater lake heating up above 30C.
The mountain's volcanic alert level remains at Level 1, but GNS scientists say volcanic tremors were "slightly elevated" towards the end of the month.
GNS described a "minor heating episode" where the volcano's crater lake temperature peaked at 35.5C on 27 December - it has since dropped to 32C, remaining hot.
They said the heating phase had caused a "gas output" with the crater lake overflowing during this time.
A number of samples were taken from the lake, which showed no changes to its chemistry.
"This indicates that there is no evidence of chemical interaction between magma (molten rock) and the hydrothermal system beneath the lake," GNS volcanologist Brad Scott said.
He said that despite the slight peak in activity, there is no reason to upgrade the warning level above green.
"The current low levels of volcanic tremor, the Crater Lake chemistry data, and peaking of the Crater Lake temperature are consistent with a low level of volcanic unrest at Mt Ruapehu.
"As a result, the Volcanic Alert Level remains at Level 1. The Aviation Colour Code remains Green," Scott said.
While the threat remains low, GNS said volcanoes like Mt Ruapehu can be unpredictable.
"The Volcanic Alert Level reflects the current level of volcanic unrest. The Volcanic Alert Level should not be used to forecast future activity.
"While Volcanic Alert Level 1 is mostly associated with environmental hazards, potential for eruption hazards also exists and eruptions," Scott said.
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