Seeds of the seagrass Posidonia australis are desiccation sensitive and as there is no seed dormancy seeds cannot be stored for use in restoration projects. To realize the restoration potential of seed-based restoration of Posidonia, this study investigated preconditioning seedlings of Posidonia in aquaculture facilities before transplanting to extend the restoration window from a few weeks (for fresh seed) to months or even years (for preconditioned seedlings). Here, we tested two levels of organic matter addition, 0 and 1.5% sediment dry weight and three sediment types; two heterogeneous sediments typical of low-energy marine environments (1) unsorted calcareous and (2) unsorted silica, and a homogeneous sediment typical of high-energy marine habitats (3) well-sorted silica. We then evaluated seedling survival, biomass and development over a period of 7 months in tank culture. There was 100% survival over the 7-month experimental period for seedlings. Seedling leaf, root, rhizome, and total biomass increased when organic matter was added to unsorted calcareous and unsorted silica sediment but not well-sorted silica sediment, although this increase was significant only after 7 months of growth. The characteristics of the sediment also influenced seedling root length and architecture. Root length and number of lateral root branches were greatest in unsorted sediments and when organic matter was present. This study demonstrates that tank culture of P. australis enabled seedlings to be available for restoration purposes for at least 7 months, and with modification of the sediment composition, larger P. australis seedlings with more substantial root systems can be produced.