Scientists believe magma high inside Mount Tongariro could trigger a bigger eruption than the one on Monday night.
GNS Science head of volcanology Gill Jolly says scientists got a good view of a long line of steam vents on the north flank of the mountain peak on Thursday.
Ms Jolly told Morning Report another team flew through the steam plume and measured sulphur dioxide and carbon dioxide suggestive of magma high up in the mountain.
She said either the volcano will go back to sleep or there will be a series of steam eruptions or a magmatic eruption similar to when Mount Ngaruahoe blew in the 1970s.
Vulcanologists also visited White Island off the coast of Whakatane on Thursday and confirmed that the volcano there has erupted, for the first time in 12 years.
Ms Jolly said both the Mount Tongariro and the White Island volcanoes were triggered by the plate tectonics off the east of New Zealand, but the actual eruptions are not connected.