New Zealand's Mount Tongariro erupted at 1.25pm local time on Wednesday (November 21) the government geological agency GNS Science said.
The GeoNet website said webcam images showed a massive plume of ash billowing into the air from the Te Maari Crater, on the west of the mountain.
Schoolchildren and hikers were walking on the mountain at the time of the eruption but the Department of Conservation said no-one was hurt.