The relation between gravity and topography in central Australia cannot be described by a conventional isostatic response function, whether it assumes local or regional compensation or whether the crust behaves as an elastic or viscoelastic medium. In particular, the response function exhibits considerable anisotropy. Hence a theory for isostatic response has been developed for a crust in mechanical equilibrium of surface loads, buoyancy, and in-plane forces and subject to erosion, sedimentation, and stress relaxation. The observed north-south response function exhibits extreme isostatic overcompensation at some wavelengths, and this can be fully explained by the viscoelastic plate model with in-plane compression. The observed east-west response function is consistent with models of regional or local isostatic compensation. The response function implies an effective flexural rigidity of (5--10)¿1022 Nm, an effective Maxwell relaxation time of 25--50 m.y., an in-plane compression of 125--150 MPa, and an erosion time constant of about 200 m.y. These parameters assume that the model evolved over a period of about 700 m.y. and are wholly consistent with the evolutionary model developed from geological observations.