A cluster of 30–52-km-deep earthquakes, and a 7–10 km step in the Moho beneath western North Island, New Zealand, are both interpreted as manifestations of active delamination of the continental lower crust and mantle lithosphere. These phenomena occur in the back-arc region beneath the east-west–oriented Taranaki-Ruapehu (TR) line, which strikes at a high angle to the present-day plate boundary through New Zealand. Across the line, there is an abrupt change in crustal and mantle lid thickness, and in upper-mantle seismic attenuation (Qp–1), showing that the mantle lithosphere has been highly thinned on the north side. We show from a receiver function profile that nearly all of the deep earthquakes reside in the uppermost mantle on the northern side of the TR line. A sum of earthquake moment tensors suggests strike-slip motion either parallel, or orthogonal, to the TR line, resulting in northwest-southeast–oriented horizontal extension. Active normal faults, oriented northwest-southeast and north-south, are seen at the surface on each side of the TR line, but the surface is uplifting here at ∼0.4 mm/yr. This requires the mantle lithosphere to be thinning at a higher rate than the overlying crust, consistent with a delamination process.