Tongaririo (c) Philip Capper, 2008, FlickR (Attribution).
On the 6th August 2012 Mount Tongariro erupted sending an ash cloud of approximately 10,000 cubic metres that was identified in Napier and Wellington.
The ARBs have several assessment resources that look at warning signs, volcanic eruption process, parts of a volcano, activity and dangers of an eruption, and the spread of ash.
Check out the resources
When a volcano erupts (PE8520)
A volcano erupts (PE8027)
Diagram of a volcano (PE8009)
Volcanic eruptions (PE7564)
Colliding tectonic plates (PE8522)
The Tarawera eruption (PE8528)
Click for further information about Mount Tongariro at GNS Science or Wikipedia.