After Mt Tongariro spewed ash into the sky, scientists are closely watching its potentially more destructive neighbour Mt Ruapehu.

The immediate threat of eruption at Mt Tongariro has passed but there remains a chance of another sudden eruption within the next two weeks, GNS Science says.

Tamatea Intermediate School's volcano studies took on unexpected realism as Tongariro erupted in front of a fifth of the Hawkes Bay school's pupils on a trek during their end-of-year camp yesterday.

Police say they would "discourage" sightseers from travelling to Mt Tongariro following an eruption at the Te Maari crater today.

These images from the GeoNet camera at Te Maari Crater show the scene changing from 11.30am to 1.30 pm today, when an eruption occurred.

Scientists are warning there are heightened signs of volcanic activity at Mt Ruapehu and an eruption could be just weeks away.

Prime Minister John Key says he was made aware a few days ago that there was increased activity on Mt Ruapehu.

A Kiwi scientist has helped discover a monster volcano under Hong Kong - one of just 50 'supervolcanoes' worldwide on the scale found under the city.

Officials are looking at reopening tracks which have been closed since the mountain's eruption.

Tongariro erupted late on August 6, throwing out rock, ash and steam. It was the first time since 1897 that it had erupted.

Only hours after Judy Bailey completed the Tongariro Crossing, the volcano awoke from its century-long slumber...

The threat of volcanic eruption has done wonders for tourism in Iceland.

Heavy rain has caused a lahar - a stream of ash and mud - from Tongariro's northern slopes to close a section of Highway 46 near Lake Rotoaira, but a GNS volcanologist says this is completely in keeping with the after-effects of eruptions.

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

Scientists, wary of another eruption, have managed to move as close to Mr Tongariro as is safely possible since Monday night's unexpected blast.