The northern half of the South Island of New Zealand lies on what plate boundary.

The northern half of the South Island of New Zealand lies on a complex region of tectonic activity that is primarily associated with the boundary between the Pacific Plate and the Indo-Australian Plate. This area is characterized by both convergent and transform boundaries, where the Pacific Plate is subducting beneath the Indo-Australian Plate, leading to significant geological features such as the Southern Alps.

As the plates interact, they create varied topographical features including mountains and valleys, as well as seismic activity such as earthquakes. New Zealand is known for its dynamic geology due to this tectonic setting. The effects of this plate boundary can be vividly seen in the landscape and have a profound impact on the ecology and human activities in the region. Thus, the northern half of the South Island is an important area for studying plate tectonics and its effects on Earth's surface.