Scientists say there is still a 50/50 chance that Mount Tongariro could erupt - and it could be at any time.

Volcanologists studying rocks and ash that were thrown up by this week's blast kept a cautious two kilometre radius from the mountain's crater.

GNS volcanologist Brad Scott told APNZ said the major 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 it could stay as it is or it could enhance.''

He said a on flight over the mountain on Thursday, scientists measured "a couple of thousand tonnes'' of sulphur dioxide gas, which indicated molten lava was bubbling inside.

"And whether or not that molten material carries on to the surface or stops where it is - we just don't have any strong indications either way at the moment.''

Indications an eruption was imminent included more earthquake activity and larger volumes of gas coming out of the crater.

Mr Scott said the best case scenario was a couple of weeks' notice 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 scientists, Mike Rosenberg, said both 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.''

Mr Rosenberg said the plume's visibility was the result of calm weather and cold air allowing the steam to rise.

Any larger eruption at Mt Tongariro was likely to be on a relatively small scale, similar to eruptions at Mt Ngauruhoe in the 1970s, GNS said.

Analysis by Massey University of ashfall taken from Mt Tongariro earlier this week found moderate levels of the potentially toxic chemical fluorine.