The ground beneath Mt Ruapehu is rumbling and the crater lake is warming.

A new episode of volcanic tremors and small earthquakes had been under way beneath the mountain since Friday, says volcano information specialist Brad Scott.

The temperature of the crater lake had also started rising a few days after reaching a relative low of 22 degrees Celsius on August 11. It's now up to 30C and the lake was overflowing.

GeoNet says the changes are within the normal range, however, and there was no need to lift the alert level.

Ruapehu remains at volcanic alert level 1, indicating minor volcanic unrest. The aviation colour code is green, indicating the volcano is in a normal, non-eruptive state.

"This is expected activity for the crater lake," says Brad.

"Since 2003, the lake has undergone 15 heating-cooling cycles and no eruptions have occurred when the lake temperature is high. The two small eruptions since 2003 were from a cooler lake."

Volcanic earthquakes being detected were small and their locations suggested they originated from shallow depth beneath the crater lake area.

The difference between volcanic earthquakes and volcanic tremor was gradational.

A sequence of discrete volcanic earthquakes could evolve into a more continuous seismic signal which would then be called volcanic tremor.

Tremor strength could vary within a few hours, or even by the minute.

"It is not unusual to observe volcanic tremor and or volcanic earthquakes during phases of increased heat input into the lake," says Brad.

Although the number of volcanic earthquakes was slightly higher, it was still within the known behaviour for previous episodes in April 2016, September 2017 and May 2018.

"These did not result in any eruptive activity. Therefore, we consider the current level of activity is within the normal range of known behaviour."

The Ruapehu crater lake became re-established in 2003, following eruptions in 1995 and 1996. It stabilised at an overflow level in 2006. Both before and after the eruptions, the temperature in the lake has ranged from about 15 - 40C.

As well as heating, the lake has also now changed colour to a grey tone.

Colour changes from pale blue were common during the heating phase as gases rising through vents on the lake floor disturbed sediments that became mixed with the lake water, Scott said.

-Stuff.co.nz