Scientists say there is a 50/50 chance that Mt Tongariro could erupt at any time.
Volcanologists studying rocks and ash that were thrown up by this week's blast kept a cautious 2km from the mountain's crater.
GNS volcanologist Brad Scott said the main finding was there was no fresh lava, only existing rock.
"It's what you'd expect to see," he said.
"There's still a 50:50 call as to whether it could stay as it is or it could enhance."
Indications an eruption was imminent included more earthquake activity and larger volumes of gas coming out of the crater.
Scott said there could be a couple of weeks' warning of an eruption "or it could be a lot shorter".
"The mountain's in an eruptive episode, it's active, it could do what it pleases."
Another GNS scientist, Mike Rosenberg, said White Island, 51km north of Opotiki in Bay of Plenty, and Mt Tongariro were at low levels of activity, with small earthquakes continuing.
"White Island continues to produce the plume that sometimes contains a bit more ash than other times," he said.
"Tongariro is about the same. People would have seen some very nice steam plumes."
Rosenberg said the plume's visibility was the result of calm weather and cold air allowing the steam to rise.
- APNZBy Rebecca Quilliam Email Rebecca