The Great Alpine Fault, which traverses from Milford Sound, up
the South Island, then through the North Island. Though it splits
into two main branches in north Canterbury.
The Canterbury side of the fault is riding up and over the
western side. The Southern Alps are one of the results of this
movement. All of the South Islands hot springs (well warm mostly)
are on the eastern side of this fault. And are considered to
because of the land uplifting faster than it is cooling.
Some modern speculation considered that this major rift is an
extension of a transcurrent fault, of the main Pacific mid ocean
ridge. This is based on the underwater topography to the south of
New Zealand.
[The main Mid Ocean Ridges all have transcurrent faults at 90
deg to the main ridge.]
There are many dozens of faults throughout the country.