Mt Ruapehu's latest bout of unrest has come to an end, with the temperature in its mountain-top crater lake reaching a peak after a month-long heating cycle.

Scientists had been closely watching Ruapehu's active vent-crater as the lake rose in temperature by a degree each day, starting at 23C last month and climbing above 40C.

But GNS Science volcanologist Brad Scott said the cycle had peaked on September 5, at 43C, and had since dropped back to 39C.

"Calculations indicate that around 600MW of heat energy was needed to heat the lake, being about the same as the heat required to generate electricity from one of our geothermal power stations."


It followed another episode last October, when, amid volcanic tremors, the lake heated at a rate of 1C per day until it reached a peak of 40C.

Earlier in 2016, the lake heated to 44C - the hottest it had been since the new lake was established post 2000 - and a tourist flight over the feature noted "vigorous steaming" and "upwelling-bubbles".

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

Since the eruptions of 1995 and 1996, crater lake temperatures have typically ranged between 16C and 38C.

By early August, temperatures had fallen to a low of 19C.

GeoNet collects temperatures from the lake using a data logger with a temperature sensor, which scientists monitor via satellite link.

An analysis of the lake temperature data since 2009, when the data logger was installed, shows that for 5 per cent of the time, it was warmer than 38C, and for a similar period could be cooler than 16C.

Its median temperature was estimated at 24C.

"Looking at the longer-term crater lake temperature data, collected from lake visits since 1950, the behaviour is similar," Scott said.

The heating and cooling cycles are controlled by a mix of volcanic and geothermal processes.

"Further sampling and visits to the crater lake are planned as the weather allows, being part of the standard GeoNet monitoring programme for Mt Ruapehu."