We investigate the influence of a crustal low viscosity zone (CLVZ) on numerical predictions of post-glacial, late Holocene sea-level variations in the Australian region. Specifically, we compare predictions from a model with a 15 km thick CLVZ within an otherwise elastic plate to results from a suite of models in which the thickness of a purely elastic crust-lithosphere is varied. We conclude that previous inferences of lithospheric thickness based on differential highstands from the region represent (approximately) the thickness of the elastic layer above the CLVZ; and thus, if a CLVZ exists, these inferences may be significantly underestimating the total depth extent of the crust-lithosphere system.