The New Caledonia Trough is a bathymetric depression 200–300 km wide, 2300 km long and 1.5–3.5 km deep between New Caledonia and New Zealand. In and adjacent to the trough, seismic stratigraphic units, tied to wells, include: Cretaceous rift sediments in faulted basins; Late Cretaceous to Eocene pelagic drape and ∼1.5 km thick Oligocene to Quaternary trough fill that was contemporaneous with Tonga–Kermadec subduction. A positive free-air gravity anomaly of 30 mGal is spatially correlated with the axis of the trough. We model the evolution of the New Caledonia Trough as a two-stage process: (i) trough formation in response to crustal thinning (Cretaceous and/or Eocene) and (ii) post-Eocene trough-fill sedimentation. To best fit gravity data, we find that the effective elastic thickness (Te) of the lithosphere was low (5–10 km) during Phase (i) trough formation and high (20–40 km) during Phase (ii) sedimentation, though we cannot rule out a fairly constant Te of 10 km. The inferred increase in Te with time is consistent with thermal relaxation after Cretaceous rifting, but such a model is not in accord with all seismic-stratigraphic interpretations. If most of the New Caledonia Trough topography was created during Eocene inception of Tonga–Kermadec subduction, then our results place important constraints on the associated lower crustal detachment process and suggest that failure of the lithosphere did not allow elastic stresses to propagate regionally into the overriding plate. We conclude that the gravity field places an important constraint on geodynamic models of Tonga–Kermadec subduction initiation.