After a century of quiet, it was the second eruption from Tongariro in four months, and vulcanologist Tony Hurst says it is unlikely to be the last.
"Certainly within the next few months there's going to be a much higher probability of these things happening," he said.
The official GNS Science monitoring service says the North Island volcano appears to have settled down for now.
Witnesses said the eruption at the Te Maari Craters sent a plume of ash high into the atmosphere.
"It was pretty spectacular. All of a sudden a towering black plume just began erupting very quickly, skyrocketing up," local resident Clint Green told Radio New Zealand.
"At first I didn't believe what I was seeing."
About 100 school students and another 50 hikers were on the iconic Tongariro Crossing when the Te Maari craters blew but there have been no reported injuries.
The aviation colour code has been changed to red, meaning flights could be disrupted.
Air New Zealand said it did not expect disruptions but it was closely monitoring the situation.
New Zealand's Civil Defence has urged people living nearby to stay indoors and close all windows if ash starts falling.
Scientists warned about increased volcanic activity in the area this week, saying that neighbouring Mount Ruapehu was in danger of erupting as pressure built in a subterranean vent.
Robyn Bennett, who lives about a kilometre from Mount Tongariro, said the volcano "just blew her stack" in the latest eruption and sent a huge black cloud over her house, giving off a strong smell of sulphur.
"It's hard to breathe if you go outside, it's pushing out quite heavily," she told Fairfax Media.
A volcanic eruption in the national park in 1953 caused New Zealand's worst rail disaster, creating a massive mudslide that washed away a bridge at Tangiwai, resulting in a passenger train crashing and killing 151 people.