There has been debate on how convergence of mantle lithosphere is accommodated during continental collision in the central South Island of New Zealand--whether by intracontinental subduction or continuous deformation of mantle lithosphere. Intermediate depth earthquakes are sparse beneath the central South Island, and abundant seismicity 130--230 km deep within the Hikurangi subduction zone to the northeast ends abruptly beneath the Nelson region of the northern South Island. This termination of seismicity has previously been interpreted as marking the southwestern edge of the subducted slab. Here we investigate the stress and strain regime at this termination of activity, and find that it is very similar to that within the subducted plate both updip and along strike towards the northeast. Our results are consistent with subduction continuing further southwest, albeit largely aseismically, as suggested by shallow earthquakes in the northeastern South Island.