NUMERICAL models of viscous mantle flow indicate that a broad subsidence should occur above subduction zones(1-4). This ’platform subsidence’ is predicted to cause an asymmetrical depression 500-1,000 km wide, with a maximum depth of similar to 2 km if filled with water. Direct evidence linking platform subsidence to mantle flow has remained elusive, however. Here we offer evidence from western New Zealand for a complete link in both space and time between subduction initiation and platform subsidence. Because of the broad wavelength (>200 km) of the observed 1,500-m subsidence it is unlikely that the subsidence is due to a flexural foredeep. The observed subsidence does, however, match in both wavelength and amplitude that predicted by a model of subduction-induced mantle flow. The occurrence of platform subsidence has implications not only for understanding mantle convection, but also for explaining geoid anomalies over active margins(5) and patterns of continental submergence(6) and sedimentation(7) that could otherwise be interpreted as a eustatic sea level change(8).