GeoNet issued a statement about 1pm today.
Scientists raised the alert level to two, which indicates moderate volcanic unrest. There are five levels in the alert system.
The crater lake continues to heat and is now 43degC, accompanied by bursts of volcanic tremor and a marked increase in the amount of gas through the lake.
GeoNet said the combination of the increased lake temperature, volcanic tremor and gas output have led to the change.
"Mt Ruapehu is an active volcano and has the potential to erupt with little or no warning when in a state of volcanic unrest. The Aviation Colour Code is changed to yellow."
Duty volcanologist Mike Rosenberg said gas output through the crater had also increased markedly, with the mix of gases in the plume the largest measures in the past two decades.
Rosenberg said level 2 was the highest warning of unrest on the mountain and people should have a higher level of awareness.
GNS Science and the National Geohazards Monitoring Centre are watching the situation closely.
It is just over a year since a fatal eruption occurred further north at Whakaari/ White Island off the Bay of Plenty on December 9, killing 21 people and seriously injuring others.
