Mt Ruapehu crater lake rises to 40C amid period of 'minor unrest'

Mt Ruapehu's crater lake temperature is continuing to fluctuate, but the government geoscience agency says volcanic activity remains within normal parameters for the mountain.

On Tuesday GNS Science said the crater lake temperature had risen to 40C, with around 400 megawatts of heat entering the lake according to recent samples.

The volcanic alert level remains at 1, indicating minor volcanic unrest.

The temperature rise follows a period of relative unrest for the volcano, after GNS raised the volcanic alert level to 2 on December 21 last year due to an "increased risk of eruption".

The volcano's crater lake had increased to a temperature of 43C, where it continued to hover until January 11, when GNS lowered the alert level to 1, saying temperatures had reduced.

Mount Ruapehu last erupted in September 2007. Photo / File

By February 15, temperatures in the crater lake had fallen further to 26C.

Although the lake was now sitting at 40C, a GNS volcanologist said all observations suggested volcanic activity currently remained low at Mt Ruapehu.

"We conducted two gas flights and collected lake samples twice. The results from these visits and our continuous monitoring of volcanic earthquake activity, tremor levels, and Crater Lake [Te Wai ā-moe] temperature and level indicate that key parameters remain within normal ranges," GNS Volcanologist Yannik Behr said.

However, Behr said Ruapehu was still classed as an active volcano, and eruptions could occur at any time.

"Mt Ruapehu is an active volcano and has the potential to erupt with little or no warning when in a state of volcanic unrest.

"The volcanic alert level should not be used to forecast future activity; however, at volcanic alert level 1, eruptions are less likely than at volcanic alert level 2."

The volcano last erupted in September 2007 without warning, with two lahars travelling down the mountain and a seven-minute earthquake recorded nearby.