Mt Ruapehu's Volcanic Alert Level has been raised from level one to two, as the active volcano experiences a period of heightened volcanic unrest, resulting in the highest gas levels of the past 20 years.
GNS Science issued the alert on Monday afternoon after what Volcanologist Michael Rosenberg said was a week of rising temperatures within the crater lake, reaching a high of 43C.
Rosenberg said the heating in the lake had been accompanied by bursts of volcanic tremor and a marked increase in the amount of gas passing through the crater lake.
In early November, GNS warned that temperatures in the lake had risen to 22C, rising sharply from the temperature of 12C recorded in September.
The temperature change is not unprecedented.
"Since 2007 Crater Lake temperature has exceeded 40C a number of times, without leading to an eruption. However, the combination of the increased lake temperature, volcanic tremor and gas output has motivated the Alert Level change," Rosenberg said.
Rosenberg said the change to Level 2 was not indicative of an impending eruption but was a measure of an increased level of unrest.
"The key thing is the high level of volcanic gases. Those gases are the highest level we've seen in the last 20 years or so," he told the Chronicle.
"We've seen these signs of unrest before, and this is part and parcel of Ruapehu being an active volcano.
"The Volcanic Alert Level reflects the current level of volcanic unrest or activity and is not a forecast of future activity.
"While Volcano Alert Level 2 is mostly associated with environmental hazards, eruptions can still occur with little or no warning."
In response to the change in alert levels, the Department of Conservation has blocked access to areas of the national park within two kilometres of the crater lake.
According to DOC, both Mount Tongariro and the Tongariro Alpine Crossing are not affected.
The volcano last erupted in September 2007 without warning, with two lahars travelling down the mountain, with a seven-minute-long earthquake recorded nearby.