The government's Institute of Geological and Nuclear Sciences ( GNS Science) said the temperature beneath Ruapehu's Crater Lake was about 800 degrees centigrade, but the lake itself was only 20 degrees centigrade, which suggested a vent was partly blocked.
That could lead to a pressure build-up beneath the Crater Lake, indicating a heightened likelihood of eruptions over the coming weeks to months.
"We think the pressure beneath Ruapehu Crater Lake has increased and this makes an eruption more likely over the next weeks to months," GNS Science duty volcanologist Steve Sherburn said in a statement.
A build-up of pressure beneath the Crater Lake was thought to have caused the last eruption in 2007 and a smaller eruption in 2006.
Since late October, small earthquakes had been occurring about 5 km beneath the summit area of Ruapehu, a popular winter skiing ground, but these might not be directly related to the high temperatures beneath the Crater lake as they were much deeper.
As a result, GNS Science had increased the aviation color code warning for aircraft from green to yellow, which reflected a change from a normal non-eruptive state to showing elevated signs of unrest.
However, the volcanic alert level remained at 1 on the scale of 0 to 5, indicating initial signs of possible volcano unrest.
In August, two other New Zealand volcanoes erupted, but no damage or casualties were reported.
Mount Tongariro, also in the central North Island, erupted on Aug. 6, throwing out small amounts of ash in its first eruption since 1897.
Two days later, White Island, a marine volcano about 50 km off the east of the North Island, erupted, sending up an ash plume 200 to 300 meters in the air, in its first eruption since 2001.
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