Tests have revealed that magma is bubbling high inside Mount Tongariro, which could suggest a larger eruption is imminent.
A series of samples have been tested since the volcano's Te Mari crater erupted on Monday night, but the latest results give the greatest insight.
The results detected sulphur dioxide and carbon dioxide in the steam plume, which indicated that magma was near the surface of the crater, GNS Science head of volcanology Gill Jolly told Radio New Zealand.
This could lead to a magmatic eruption, but it's also likely that a series of steam eruptions could follow, Jolly said. Or perhaps, nothing at all.
The volcano was still ejecting steam and gas this morning, GNS volcanologist Michael Rosenberg said.
Tremors continue to shake the earth below it, he said.
GNS Science says a minor amount of ash is visible from some steam vents, which are clearly visible today as the weather is clear.
Scientists today are undertaking further visual observations and will be collecting gas and water samples from the nearby Ketetahi hot springs.
Meanwhile, a 4.7 magnitude earthquake in the Bay of Plenty this morning has had no impact on White Island, which erupted on Tuesday night.
It was the first eruption in 12 years for the country's most active and largest cone.
White Island tended to have volcanic episodes which lasted a few
months to a few years, so this could just be the start of more to
come, Rosenberg said.
The Te Mari crater last erupted in 1897.