High levels of carbon dioxide from Mt Ruapehu have prompted a warning from Government scientists.

GeoNet has issued a low-level warning after readings last week at Crater Lake showed the largest CO2 emissions in recent years.

Fine weather allowed scientists to make airborne gas measurements which recorded not just high levels of carbon dioxide, but also sulfur dioxide and hydrogen sulfide emissions.

Scientists say they are not worried but have issued a level one alert - the second-lowest possible - saying the monitoring data suggests the open vent volcano is degassing - which has been observed in the past outside eruptive periods.

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The lake temperature was 37 degrees celcius, which was near the top of its usual range. Volcanic seismic tremor remained at moderate levels.

"This is something that happens every few years, we get higher temperatures and higher gas concentrations, but it's been sustained for a certain amount of time so we thought we would inform the public," duty volcanologist Tony Hurst said.

GNS said it would monitor the mountain closely as well as analyse further samples.