The AUSGeoid98 geoid model is used in conjunction with nation-wide GPS networks and spirit-levelled Australian Height Datum (AHD) heights at 1013 points, and subsets thereof, to estimate the vertical offset between the AHD as realised on the Australian mainland [AHD(Mainland)] and in Tasmania [AHD(Tas)]. It appears that the AHD(Tas) is offset below the AHD(Mainland) somewhere between (26±33)cm and (12±12)cm, which is in broad agreement with previous estimates made by Rapp (1994) and Rizos et al. (1991), as well as with long-wavelength sea-surface topography models. However, the estimates of the vertical datum difference are heavily influenced by the spatial distribution of the GPS-AHD control points used, with different offsets being computed according to latitude and proximity to the coast. This is most probably due to the effect of fixing of multiple tide gauges to mean sea level in the adjustment of the AHD. This has implications for any future unification of the AHD(Mainland) and AHD(Tas) into a single, global vertical datum, where the appropriate datum connection points must first be chosen.