The global demand for Cobalt (Co) over the next decade is expected to increase more than any other critical mineral due to its key role in renewable technologies such as lithium-ion batteries. Australia holds the second largest Co deposits worldwide and is the second largest producer. The subsequent mining, processing and transport activities raise concerns of Co contamination to coastal waterways, however, our current understanding of Cobalt behaviour in estuarine systems is limited. This study investigated the behaviour of Co during transport through an urban estuary on the subtropical east coast of Australia (Pimpama River, Queensland). Benthic sediment samples were collected along the river transect and analysed for Co, Iron (Fe), Manganese (Mn) and Aluminium (Al) concentrations. The spatial distribution of Co concentrations in the sediments generally decreased with distance downstream. Elevated concentrations at sites aligning with agricultural drains indicated that disturbance of acid sulfate soils is the main source of Co to the river sediments. Cobalt was positively correlated with Fe, Mn and Al, suggesting that iron/manganese/aluminium (hydr)oxides are potential sorption sites for estuarine cobalt. Sediment resuspension during simulated tidal inundation showed that Co was readily released from the particulate phase to the dissolved phase by competitive cation displacement. The dissolved Co concentrations released during seawater resuspension were over twice the ANZECC marine 95% species protection trigger value. Furthermore, the more bioavailable and highly mobile dissolved phase dominated the Co concentrations across a broad salinity range (1-32) during laboratory mixing experiments. This study provides information regarding the transport mechanisms of Co that, due to increased demand and rising sea levels will require ongoing environmental management in coastal waterways.