Mount Ruapehu’s crater lake (Te Wai ā-moe) is warming but appears to be running through its usual heating and cooling cycle.

While the volcanic alert level was at one and the aviation colour code at green, it paid to remember it was still a volcano in unrest, said GNS Science duty volcanologist Yannik Behr.

“It means, as we always say in our alert bulletins, eruptions can happen any time. We’ve had blue sky events in the past, back in 2007 for example.”

In its July 12 update GNS reported that another heating phase had started.

Over the last three months the lake temperature had cooled from 41 degrees centigrade on 2 April to 20 degrees on 2 July, the bulletin said.

The turnaround of the temperature in early July followed two small volcanic earthquakes and was accompanied by two days of a higher level of volcanic tremor as was common at the start of a heating episode.

​GNS, together with the Department of Conservation, performed labelled tests three times a week of the eruption and lahar early warning system, said Behr, but also end-to-end tests several times a year.

Run from the GNS facility in Avalon these involve replaying signals that happen during an eruption and then checking the system had recognised them.

The most recent end-to-end test was on Thursday morning.

Credit: Stuff.co.nz