Crater Lake on top of active volcano Mount Ruapehu has been bubbling this afternoon as temperatures rise to the hottest on record.

Around 2pm a tourist flight over the Crater Lake observed "vigorous steaming" at the lake and disturbance of the surface itself with "upwelling-bubbles".

This sort of activity has not been seen in years while Crown research institute, GNS Science, says a temperature of 44 degrees is "the hottest lake temperature we have recorded since the new lake was established post 2000."

A swarm of volcanic earthquakes reported on Tuesday has now stopped, however, seismic network at Ruapehu continues to record volcanic tremor.


"The level of volcanic tremor has increased slightly this week, but is at levels lower than those reached in April 2015 when the lake was also heating," Brad Scott, volcanologist said.

"Volcanic tremor is always present at Ruapehu, and the level does vary a lot. Research suggests volcanic tremor is linked to both hydrothermal and magmatic processes at Ruapehu and not necessarily an indication of an upcoming eruption."

Crater Lake's temperature appeared to have reached a maximum on May 3 at 42 degrees, however it has risen further to 44 degrees. GNS Science says it is monitoring the situation and will provide more information as it comes to hand.