An aerial observation was being conducted over Mt Ruapehu yesterday after temperatures in the active volcano's bubbling Crater Lake rose to the hottest on record.

On Saturday, a tourist flight over the Crater Lake observed "vigorous steaming" at the lake and disturbance of the surface itself with "upwelling bubbles".

This sort of activity had not been seen in years while Crown research institute GNS Science said a temperature of 44C was "the hottest lake temperature we have recorded since the new lake was established post-2000".

A flurry of volcanic earthquakes reported on Tuesday had now stopped. However, the seismic network at Ruapehu continues to record volcanic tremors.

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GNS volcanologist Brad Scott had been closely monitoring activity overnight on Saturday and said there had been only more tremor and an insignificant change in lake temperature of around 0.3 of a degree.

"There has been little or no change from yesterday - activity has been dominated by volcanic tremors with no distinct or individual volcanic earthquakes."

But the elevated temperatures did not necessarily mean an eruption was imminent.

"In fact, if you track back through the record, sometimes it has erupted from cold lake temperatures. This is what makes it difficult for us, as there's no fixed pattern.

"In the end, it's an active volcano and we should always pay respect to it."

The developments weren't enough to upgrade Mt Ruapehu's volcanic alert level, which remained at 1.

The last time it erupted was on September 25, 2007, causing a 7 minute earthquake, two lahars and flying rocks - one of which crushed the leg of primary school teacher William Pike when it landed on Dome Shelter near the crater.

Since then, there have been warnings in 2008, 2011, 2012 and this year - none of which resulted in another major event.

Meanwhile, unrest was continuing at the volcano at White Island, where a minor eruption occurred last month. While there hadn't been another event as large, Mr Scott said, small geysering at the crater and a series of volcanic earthquakes indicated there was still "significant unrest" at the offshore volcano in the Bay of Plenty.