GeoNet has issued a Volcanic Alert Bulletin for Mt Ruapehu, as the temperature in the crater lake continues to rise.

Scientists have raised the alert level to two which indicates moderate volcanic unrest.

The crater lake continues to heat and is now 43C, accompanied by bursts of volcanic tremor and a marked increase in the amount of gas through the crater lake.

GeoNet in a statement said the combination of the increased lake temperature, volcanic tremor and gas output have led to the alert level 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.

"While Volcano Alert Level 2 is mostly associated with environmental hazards, eruptions can still occur with little or no warning."

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.

He said Mt Ruapehu was an active volcano with the potential to erupt with little or no warning.

GNS Science and the National Geohazards Monitoring Centre continue to watch the situation closely.