Monday's Tongariro eruption could be the start of a multitude of larger eruptions, spewing even more ash into the air, a disasters and hazards expert says.
Dr Thomas Wilson, lecturer in hazards and disaster management at Canterbury University, says activity on the mountain could change quickly.
However, it is difficult to know what will happen next and although things could get worse the volcano might settle down completely or small eruptions could continue.
"I can't give you any probabilities on the likelihood of these scenarios," he said.
"But we can't rule anything out at this stage."
The volcano erupted at 11.50pm, spewing ash from the Te Maari craters on the northern side of the mountain.
It has disrupted flights over the central North Island, and aerial news footage shows a fine covering of ash downwind from the volcano.
What happened in southern Japan following the eruption of the Shinmoedake volcano in 2011 is similar to what would happen in New Zealand if a much larger eruption was to occur at Tongariro, Dr Wilson says.
Centimetres of ash covered farms and towns, impacting on agriculture but it took just six months for things to return to normal.
Some residents had to use face masks and avoid going outdoors at times, and power and water supplies were also affected.
Dr Jan Lindsay, senior lecturer and volcanologist at Auckland University, says it is possible ash may reach Auckland as this has happened in the past.
"It will be interesting to see how the eruption develops - whether it continues for months, or is over already," she said.
"If the former we may see ash reaching Auckland, depending of course on the wind direction."
Tongariro - what could happen next?
* Activity stops
* Small eruptions continue with light ash falling across the North Island
* Larger eruptions with thicker amounts of ash fall across a large area.
Source: Dr Thomas Wilson, lecturer in hazards and disaster management, Canterbury University.
NZN