Steam plumes have been seen above Mt Ruapehu this morning, as the crater lake has heated up close to its natural peak, GNS Science says.
The lake began heating early this month, rising from 13C to its current level of about 37C.
No seismic or acoustic activity has been recorded, indicating the steam was not created by volcanic activity, but rather the combination of the cold atmosphere mixing with the hotter crater lake air, volcanologist Dr Geoff Kilgour said.
"It's probably more an atmospheric effect. Essentially you've got relatively cold, ambient air above the volcano, and then you've got a lake which is almost 40C and is heating up the atmosphere around it. It essentially causes cloud formation."
Kilgour said the lake naturally moved through heating and cooling cycles every nine to 12 months, and would likely reach its normal upper limit of 40C soon before beginning to cool.
The volcanic alert level for Mt Ruapehu remained at level one, indicating minor volcanic unrest.