GNS Science says early testing of ash from Mount Tongariro has confirmed the eruption on Monday was steam-based.
Masey University conducted preliminary tests on the ash, which shows moderate levels of fluoride, similar to that seen in the 1995 Mt Ruapehu eruption.
GNS Science says the ash samples also showed there was little magma ejected from the eruption late on Monday night.
Head of Vulcanology Gill Jolly told Morning Report that means the eruption was caused by gases and heat coming from below.
Vulcanologists hope to fly over the Mt Tongariro again on Thursday to collect more samples.