The Institute of Geological and Nuclear Sciences said the temperature beneath Ruapehu's Crater Lake was 1,400 degrees Fahrenheit but the lake itself was only 68 degrees F., suggesting a volcanic vent may be partly blocked.
A resulting pressure build-up beneath the Crater Lake would mean a heightened likelihood of eruptions in the near future, experts said.
"We think the pressure beneath Ruapehu Crater Lake has increased and this makes an eruption more likely over the next weeks to months," vulcanologist Steve Sherburn said in a statement.
Such a build-up of pressure beneath the lake was thought to have triggered the last eruption of Ruapehu in 2007 and a smaller eruption in 2006, Chinese news agency Xinhua reported.
In August, two other New Zealand volcanoes erupted.
Mount Tongariro, also in central North Island, erupted Aug. 6 for the first time since 1897.
Two day later White Island, a marine volcano about 30 miles to the east of North Island, erupted, sending up an ash plume aloft in its first eruption since 2001.