The appearance of steam from Mt Ruapehu's crater is nothing to worry about, according to GNS Science.
The crater lake has reached 40deg, the warmest temperature recorded since 2003.
The Crown research institute says the lake is approaching the peak of a heating cycle, the eighth since the lake re-formed in 2002 following eruptions in 1995 and 1996.
It says plumes of steam may be visible from the Desert Road, though the volcanic alert level remains at one, the lowest possible level.
GNS Science says the warming of the lake is not connected to the earthquake in Christchurch.