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.