Mt Ruapehu's Crater Lake shows signs of cooling

Earlier this month, the mountain-top lake was measured at 46C - its highest temperature in over a decade. Photo / Maurice Costello

After a period of uncharacteristic bubbling and steaming, Mt Ruapehu's Crater Lake is showing signs of cooling down.

But scientists have pointing to a range of other indicators suggesting the volcano is still in a state of "heightened unrest".

Earlier this month, the mountain-top lake was measured at 46C -- its highest temperature in over a decade -- while an observation flight found it to be steaming.

Since then, the temperature has dropped to 39C, although some of this may have been due to rain and snow falling into the lake, GNS Science volcanologist Dr Gill Jolly said.

Meanwhile, moderate levels of volcanic tremor were continuing and recent visits to the volcano had also measured an increase in the output of volcanic gas.

"When the weather conditions allow we will be repeating these to see if this continues."

An analysis of water samples from the lake showed little change from previous samples, which implied the lake geothermal system had not responded to the recent increase in gas and heat through it.

The volcanic alert level for Mt Ruapehu remained at 2 -- signifying "heightened unrest" -- while the aviation colour code would also stay at yellow.

The lake temperature typically ranges between 15C and 40C, something which has been a common feature since it reformed between 1999 and 2000, having been removed by the 1995-1996 eruptions.

The last time it 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 then, there have been warnings in 2008, 2011, 2012 and this year -- all of which did not result in another major event.