Cainozoic ostracod palaeodepth indices from five middle bathyal DSDP sites, including 209 (Queensland Plateau) 207, 208, (Lord Howe Rise) 593 (Challenger Plateau) and 277 (Campbell Plateau) in the southwest Pacific demonstrate a history of progressive submergence of each site. Deepening from shelf to upperbathyal depths is shown to have occurred first in the Early Eocene at Site 277, in the middle Eocene at sites 207 and possibly 208, in the Early Oligocene at Site 593 and in the Early or Middle Miocene of Site 209 indicating nonsynchronous foundering of adjacent continental crustal regions of the Tasman Basin. Tectonic readjustments after the cessation of Tasman Ridge spreading in the Late Palaeocene is considered to be the primary factor controlling palaeodepth changes of the eastern Tasman Basin margin. Present day middle bathyal depths were reached in the middle Eocene at Site 277, Late Oligocene of sites 593 and probably 207 and 208, and in the Early or Middle Eocene of Site 209. Sites 277 and 593 record the Eocene/Oligocene boundary and neither site displays major changes in ostracod composition despite previously reported global palaeoceanographic changes documented at that time. A large increase in ostracod valve size in the middle or Late Oligocene of Site 593 is likely to be a response to temperature decrease. This observation is consistent with the previously proposed scenario (Kennett et al. 1972) that invoked the first influence of Circum-Antarctic Current upon Tasman Sea circulation occurred in the Oligocene.