Marine crustal multichannel seismic data, recorded offshore South Island, New Zealand, are used to provide constraints on the evolution of the Gondwana margin in this region. Lower crustal and upper mantle seismic reflectivity off southeastern Stewart Island, south of South Island, is interpreted in terms of a northeast dipping paleosubduction zone. This inferred suture is considered to be related to the docking of the Brook Street oceanic island arc terrane to the Gondwana margin in the Triassic, the junction of which is now largely covered by the Median Batholith. A similar lower crustal--upper mantle feature has been imaged off western South Island. Both inferred sutures have similar strikes and are approximately collinear after restoration of the region for major Cenozoic horizontal deformation. It is proposed that they are parts of the same paleosuture that was oriented subparallel to the former Gondwana margin and to the present Paleozoic and Mesozoic terranes forming South Island.