Mt Ruapehu’s status is now green which means the volcano is in normal, non-eruptive state.
To the left of the picture is the Australian Plate and to the right is the Pacific Plate. The Pacific Plate comprises very heavy oceanic crust; it wants to sink. And it is in fact subducting beneath the lighter continental Australian Plate, particularly in the north. The south is a slightly different story because all plates are not created equal and actually comprise both types of plate. But we aren’t interested in the South Island – as interesting as it is with the fabulous Southern Alps.
The triangles note the movement direction of the tectonic plate. To the left of the plate boundary you see a double line parallel with the boundary extending from the Ruapehu complex right up into the Pacific. This is the Kermadec Trench which morphs into the Tonga Trench. As the Pacific Plate dips into the mantle beneath the Australian Plate it catches the bottom of that plate and pulls on it, fracturing it. Thus this part of the crust is being torn apart. The whole land area that is being ripped apart is referred to as the Taupo Volcanic Zone (TVZ). The rates of pull are different, being 2-3mm per year at the Ruapehu end and about 5mm per year at the White Island end.
You can identify the Ruapehu complex as the green spot at the bottom of the rift.
Just note how big the continental area around New Zealand is. The light piece coming from the right in the South Island is the Chatham Rise.