Macquarie Harbour is a shallow fjord located on the West Coast of Tasmania and is one of the largest salt-wedge estuaries in Australia. Adjoining the South-West World Heritage Area and terminus for the Gordon, Franklin and King rivers, the harbour is a rich ecological bastion and home to the critically endangered Maugean Skate. Typified by low levels of dissolved oxygen at depth, understanding the mechanisms and stability of the harbour’s oxygen replenishment and drawdown cycles is key to assessing the impact of anthropogenic inputs on environmental sustainability and management. In this talk, sub-km-resolution coupled biogeochemical and hydrodynamical modelling is used to investigate details of the harbour’s oxygen budget — highlighting the role of tidally-pumped ‘renewal’ events in transporting oxygen-rich marine waters into the harbour at depth, and biogeochemical remineralisation processes effecting drawdown. In addition to this leading-order balance, the impact of fast, event-scale processes associated with river flows and surface winds is explored and mapped onto the occurrence of transient low-oxygen events contained in the observational record. Links between these slow and fast oxygen cycles and overall ecosystem sustainability are highlighted.