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Scientists are monitoring Mount Ruapehu after an increase in volcanic activity has been detected.
It is in a state of minor unrest, with the constant tremor beneath the volcano having increased from a weak to moderate level.
Geologist Michael Rosenberg said it could be an early warning sign of an eruption.
There had been a gradual increase in the volcanic tremor since 22 July, Rosenberg said, which had reached a level that had not been recorded in at least two years.
"It doesn't, by itself, mean that the volcano is going to erupt.
"It's more like a possible early warning signal."
Rosenberg said there had been a minimal change in temperature at Crater Lake, or Te Wai ā-moe, which remained stable.
The output of gas emissions, such as volcanic sulphur dioxide and hydrogen sulphide, were still at a low-to-moderate amount.
Rosenberg said the tremor was also within the range expected during minor unrest.
He said there was no link between New Zealand's heightened seismic activity, and an increase in the level of volcanic tremor beneath Mount Ruapehu.
The Volcanic Alert Level was at one, and the Aviation Colour Code was green.
