Civil Defence has issued a national advisory warning light volcanic ash falls can be expected in Hawke's Bay following an eruption at Mt Tongariro.
About 100 students from Tamatea Intermediate School were on the Tongariro Track when the volcano erupted, but are safe and well.
GNS Science said an ash cloud blew about 2km above the mountain, but was believed to only consist of ash and gas with no major rocks.
A woman who lives near the mountain said sulphur-smelling ash had been raining down on her home.
Robyn Bennett, who lives 6km from the mountain, said she could see the plume of ash rising into the sky.
``It's sitting under some cloud and that's why it's pushing down onto us, she said.''
``It smells worse than rotten eggs.''
Ms Bennett said the eruption was silent.
She said she spotted a vent today that wasn't there yesterday.
She didn't think she and her husband would need to evacuate, ``not unless she starts spewing out a whole lot of red rocks'', but they were waiting to hear from Civil Defence or the Department of Conservation.
Ms Bennett was not worried about the eruption.
``Why worry? You haven't got time to worry at this stage.''
The ash was drifting south east towards Mt Pihanga, she said.
The eruption came from Te Maari crater, on the western side of the mountain.
A massive plume of ash could be seen coming from the mountain, and GNS Science duty volcanologist Nico Fournier told APNZ the eruption happened about 1.20pm.
"There was one eruption, essentially one explosion, and it was not sustained. It spewed some ash possibly a couple of kilometres up the air. The ash is now drifting to the east-southeast.''
"Light volcanic ash fall can be anticipated downwind of Tongariro and may impact the following areas/regions: Waikato, Hawkes Bay, Gisborne, Bay of Plenty."
If experiencing ash fall, people living in or near the affected areas should stay indoors.