The aviation alert level around Mount Ruapehu has been raised as the volcano in the central North Island shows signs of activity.
The volcano surveillance co-ordinator for GNS Science, Brad Scott, says the temperature at the crater lake is only four degrees shy of its highest ever recorded level of 42.5°C.
Mr Scott says there have also been increases in seismic activity, changes in lake chemistry and increases in carbon dioxide emission which have never been seen before while the lake is this hot.
He says the changes show the volcano is more active than usual and warrants raising the Aviation Colour Code to a yellow alert, which means airlines should be aware of the risk of an eruption.
Mr Scott says the alert level will not be raised further unless an eruption is imminent.