It's believed to have erupted just before midnight last night, raising the volcanic alert for Mt Tongariro to a 'code red'.
Ash is covering a large part of the central area of the North Island, forcing road closures and flight disruptions.
Speaker:Michael Rosenberg, volcanologist, GNS Science, New Zealand
ROSENBERG: It's been an exciting night here.
COUTTS: Alright, describe it for us?
ROSENBERG: Well first I heard was a call shortly before midnight last night from somebody saying that they'd had reports of ash falling, and I had trouble believing that, but sure enough when I checked on the seismic records, yeah we were in an eruption.
COUTTS: Now it's more than a century so you had absolutely no notice that this eruption was about to take place?
ROSENBERG: Well we did have some warning signs. Over the last three or four weeks we have been recording very small volcanic earthquakes underneath the mountain. But in the last week or so there have been far fewer of those earthquakes and much, much smaller, and from that very low level suddenly we're into an eruption. So certainly unexpected.
COUTTS: Now it's raised to a code red, what does that mean?
ROSENBERG; Well in New Zealand we operate volcanic alert level system, which tells us about the current state of the volcano. And we also join in with the international aviation colour code system. So what we've done is raised the alert level to level 2, which means there's a minor eruption in progress, and we've raised the aviation colour code to red, which is the highest level, and this is effectively an advisory for the aviation industry that they might expect ash to be in the aviation airspace above the mountain.
COUTTS: Well what's happening now? Is it still spewing molten ash at the moment?
ROSENBERG: Well it seems like at least for now things are pretty quiet. The initial ash eruptions probably lasted less than a couple of hours. So at the moment things are fairly quiet. What that means we'll just have to wait and see.
COUTTS: Well how unusual is it that a volcano can be dormant for more than a century and then just rear its ugly head now?
ROSENBERG: Well it's certainly pretty unusual. We have some other active volcanos in the centre of the north island, and those ones erupt a lot more frequently. So yeah, it's certainly made us take notice when there's signs of activity underneath something that's been dormant for that amount of time. Short lead-in times to eruptions do happen, it's one of the reasons that our geo-net project as part of GNS Science, that's why we monitor the volcanos so that we can see these early signs.
COUTTS: Now is it in a populated area, are any of the local residents been affected by it?
ROSENBERG: It's in a sparsely populated area but it's on the edges of one of the largest national parks and a world heritage area. So there's certainly visitors there. And a matter of 20 kilometres to the south the main ski fields of the north island. There hasn't been any impact on those, but it's possible. In terms of local population, the area is fairly sparsely populated, there are certainly some local residents there, and some of those people have been affected by ash, a few centimetres of volcanic ash close up to the mountain. But at this stage the area affected by ash is quite small. There has been light ash fall right out on the east coast of the north island, so in the order of at least 150 kilometres away. But really that's very light ash and at the moment we're just seeing steam coming out from the vents.
COUTTS: So your instruments are telling you that this won't be a real problem?
ROSENBERG: Well it's really hard to know, it's a natural system so you can't predict what's going to happen. What our instruments are telling us at the moment is that certainly earthquake activity's pretty quiet, and whether it'll stop there or whether it will escalate into another larger phase of the eruption, yeah all we can do is keep watching those seismometers and wait and see.