Scientists have issued a volcanic alert bulletin, warning of minor volcanic unrest behaviour at Mt Ruapehu.

GNS Science says the crater lake (Te Wai ā-moe) temperature at Mt Ruapehu is rising along with an increase in volcanic tremor. Last week the lake starting heating again at a rate of around 1degC per day.

"Coincident with the increasing lake temperature, we have also noticed that the level of volcanic tremor has increased," duty volcanologist Geoff Kilgour says.

"Previous heating cycles have shown this increased tremor to last for days to weeks."

The Volcanic Alert Level remains at 1, and the Aviation Colour Code remains at Green.

"It is a useful reminder that eruptions can occur with little or no warning," Mr Kilgour says.

"GNS Science continues to closely monitor Mt Ruapehu and our other active volcanoes."

Newshub.