The volcanic alert level for Ruapehu has been increased from level 1 to level 2 due to an increase in the output of volcanic gas from Mt Ruapehu's Crater Lake.

Wanganui District Council emergency manager Matthew Smith said GNS Science advised of the change yesterday.

"It is important to emphasise that an increased level of unrest is not yet an eruption of Mt Ruapehu and does not mean an eruption will occur," he said.

"GNS will continue their monitoring of Mt Ruapehu and at the local level, Whanganui Civil Defence Emergency Management will review volcanic hazard plans while the duty officer continues to receive any updates."

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Mr Smith said that in the event of a very large eruption the possible hazard for Whanganui district was volcanic ash fall, but the prevailing wind direction would take ash from Mt Ruapehu towards the East Coast. GNS duty volcanologist Geoff Kilgour said the temperature of Ruapehu's Crater Lake had been rising since mid-April and for the last couple of days had ranged between 45C and 46C.

"The temperature is measured by a data logger that was installed about five years ago and what we have noticed over that time is that the temperature rises quite rapidly and takes quite a while to cool down," said Mr Kilgour. "The temperature usually peaks at around 40C before it starts to cool down."

Mr Kilgour said there had been varying levels of volcanic tremor at Mt Ruapehu since the volcanic earthquakes in late April. Although the level had increased it was not exceptional in terms of the last few years, he said.