This thesis aims to explain the effect of organisational culture on adaptation decision-making. It examines the influences of culture on the adaptive decision-making capacity of four case study organisations interacting within the Reef Water Quality Protection Plan (Reefplan) for Australia’s Great Barrier Reef. It finds the mix, strength and persistence of adaptive decision-making practices is strongly influenced by organisational culture. It concludes that organisational cultures will continue to influence Reefplan’s effectiveness by mediating the adoption and sustainability of adaptive practices.