A volcanic plume from the Tongariro eruption could contain traces of toxic chemicals but scientists say the hazard from the ash cloud is considered low.
The plume shot some 7000m into the air after the eruption about 11.50pm yesterday before dropping in altitude this morning.
It caused light ashfall to the north and east of the volcano and formed a volcanic ash cloud that stretched east across the central North Island today, with ashfall reported as far away as Napier.
Flights were delayed or cancelled this morning due to the threat of volcanic ash to aircraft, but the aviation colour code has since been downgraded from red to orange.
Civil Defence this morning warned the ash could be a health hazard, but this afternoon said the volcano was no longer producing ash and its volcanic threat advisory had been cancelled.
GNS Science said volcanic ash could be damaging and disruptive, but it was fortunate the ashfall was relatively light and mostly confined to the Tongariro National Park and Desert Road area.
Duty volcanologist Michael Rosenberg said the volcanic plume was white by the time of an observation flight over Tongariro this morning, which meant there was very little or no ash in it.
It was difficult to see the plume this afternoon due to the "terrible'' weather, but the plume was now mainly steam, and the ash hazard was considered to be low.
"There's no ash coming out at the moment and there hasn't been since early this morning,'' Mr Rosenberg said.
"Obviously with further distance from the volcano it just disperses more and more, and gradually just becomes part of the atmosphere.''
Mr Rosenberg said the ash cloud would have contained fragments of volcanic ash - tiny particles of fragmented rock about 2mm in diameter - as well as droplets of water and probably acid.
The cloud would also have contained volcanic gases like carbon dioxide and sulfur dioxide.
Mr Rosenberg said the ash cloud after the 1995 Ruapehu eruptions contained toxic chemicals like fluorine.
"Fluorine is very toxic to stock and if there was enough in the ash accumulating on people's roofs where they collect drinking water, that would also be a significant health hazard,'' he said.
But it was not yet known if that was the case with the current eruption.
"We haven't yet been able to analyse any of the ash samples that we've had passed to us so we don't know if there are those chemicals in this ash.''
Scientists were hoping to know more about the components of the ash by tomorrow, while a full chemical analysis could take a few days.
Civil Aviation Authority meteorology manager Peter Lechner said the plume was now sitting over the volcano and to the east towards Hawkes Bay.
"So that block of plume is just quietly drifting away to the east where it will be right off the coast later this evening,'' he said.
WeatherWatch said light north to northwest winds were expected tomorrow, followed by light west to southwest flows on Thursday.
The downgrade of the colour code - which is used to alert aviation operators to activity around a particular volcano - was because there was less ash in the air and the plume was much smaller.
Orange means the volcano is experiencing heightened unrest with increased likelihood of an eruption, while red means eruption is forecast to be imminent.
GNS Science said there was the potential for more Tongariro eruptions to produce more ash, and people should be aware of the hazards.
Volcanic ash was made of fine particles of volcanic rock, less than 2mm in diameter, and was very different to ash from a fire.
It was usually non-toxic to people, but could be irritating - mostly to the eyes and lungs - and could exacerbate existing respiratory conditions.
Volcanic ash generally did not produce long-term health effects.
Civil Defence earlier warned people in affected areas to stay indoors as volcanic ash was a health hazard - especially to people who suffered from breathing difficulties.
"If people are caught in volcanic ashfalls, it's important they wear a dust mask or a handkerchief over their nose and mouth,'' a spokesman said.
People caught outside in ash were advised to seek shelter and to close all windows and doors inside.
People who relied on rainwater for drinking were advised to disconnect their pipes if ash had already fallen in their area, or if ash from further activity was likely to fall in their area.
Drainpipes and downspouts from gutters should be disconnected to stop drains clogging, but allow ash and water to empty from gutters onto the ground.
