GNS Science is warning that intermittent tremors coming from Mt Ruapehu are at a level last seen during the 2006 eruption and 2007 lahar.
Volcanic tremors coming from Mt Ruapehu over the last 2-3 weeks were "moderate to strong", GNS said in a release, and were some of the strongest measured in the last eight years.
"The signals are similar to those in 2006 and 2007 but weaker than those recorded in 1994/1995."
It cautioned that, historically, a direct relationship between volcanic tremors and eruptions had not been established, but said it was a signal often present before, during or after enhanced volcanic unrest.
Mt Ruapehu remains at Volcanic Alert Level 1 (minor unrest) with the scale ranging from 0 to 5, and GNS Science continues to closely monitor the volcano through GeoNet.
The 2006 eruption sent a plume of water 200 metres into the air with waves of up to 6m hitting the walls of the crater lake.
The 2007 lahar took place after a wall of the crater lake composed of volcanic material gave way, sending an estimated 1.4 million cubic metres of mud, rock and water down the Whangaehu River. No one was injured but roads were closed and one property was cut off after the lahar destroyed its access road.
A spectacular eruption in 1995 sent a huge plume of ash high into the air and the crater lake was emptied. Air travel was disrupted and surrounding areas were blanketed in volcanic ash.