In the North Island and the northern South Island, the Pacific Plate dips below the Australian Plate. This process is known as subduction, and earthquakes originating within the subducted Pacific Plate occur as deep as 600 km beneath Taranaki. In Fiordland and the region to the south, the situation is reversed, with the Australain Plate subducting beneath the Pacific Plate. Between these two subduction zones, the crust of both plates is too buoyant to subduct, so the convergence is accomodated by a combination of uplift, which creates the Southern Alps, and movement along the Alpine Fault. Convergence of plates is largely driven by the sinking of the relatively dense deeper part of the subducted plates into the earth's mantle.
Earthquakes also result from bending and tearing stresses which occur as the plates subduct.
If the interface between the subducted and overlying plates were smooth and well lubricated, such earthquakes within the subducted plate would be all that we would experience at our subduction zones. However, this is nearly always not the case, and the plate interface periodically locks up. This has two main consequences. First, the overlying plate deforms as convergence continues, resulting in faulting in the brittle crust. Good examples of such faulting are the Wellington, Wairarapapa and Mohaka faults in the overlying plate.