Mt Ruapehu's crater lake - Te Wai ā-moe - typically goes through heating cycles, and GNS measurements show that early this month the temperature started building.
The mountain is currently at volcanic alert level 1 - the lowest of 5 alert levels.
GNS volcanologist Agnes Mazot said what's happening on the mountain is typical for this time of year.
"Level 1 means its minor volcanic unrest it's the typical level for Mt Ruapehu at the moment and we're expected to have a normal increase of lake temperature in the near future," she said.
Routine monitoring of the crater lake at Mt Ruapehu has been ongoing for the last 10 years including regular gas measurements and crater lake water samplings.
"We have recorded cycles with lows of 15 degrees and highs of 45 degrees, it's annual, this is typical behaviour within the cycle."
Mazot said the crater lake Te Wai Wai ā-moe is at the start of a heating cycle and that usually coincides with an increase in gas emission.
She said there is currently low risk of the lake level rising.
"We also measure the lake level at the same time as the lake temperature and we have an outlet so there is no risk of the lake going higher than the level until it switches water outlfow and goes to the Whangaehu River."