Scientists are warning the crater lake on Mount Ruapehu is the hottest it has been for two years.

Photo: RNZ / Kate Newton

GNS Science said today Te Wai ā-moe reached 42 degrees Celcius yesterday after rising by 0.5C per day for the last two weeks.

Only 1 percent of post-2009 temperatures had exceeded 42C.

GNS said the alert remained at level one - "minor unrest" - but warned eruptions could occur with little or no warning.