A series of volcanic earthquakes has been recorded beneath Mt Ruapehu.

GNS Science volcanologists say the earthquakes have been happening regularly since March 15 but were of relatively small magnitudes.

The could not be located by GeoNet's automatic earthquake detection system but the pattern of seismic recordings indicated a source beneath the summit area, duty volcanologist Yannik Behr said.

He said volcanic earthquakes were a sequence of discrete volcanic earthquakes but they could evolve into a more continuous seismic signal which would then be called volcanic tremors.

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Behr said the crater lake was also warmer than usual and had been at an elevated temperature since August. The lake was currently sitting at 39C - only 5 per cent of temperature measurements made since 2009 were equal or greater than that.

It was not unusual to see volcanic earthquakes during phases of increased heat input into the lake, he said.

"Similar episodes were observed during April 2016 and September 2017 but did not result in an increase in volcanic unrest. The Volcanic Alert Level, therefore, remains at 1 and the Aviation Colour Code at Green."

GNS Science would continue to closely monitor Mt Ruapehu.