South-western Australia is a climate change hotspot with reduced rainfall and increased temperatures. Estuaries are experiencing reduced freshwater inflow and longer periods of high salinities, which affect biodiversity and trophic interactions. Black Bream (Acanthopagrus butcheri) is a long-lived sparid of cultural and fisheries significance. Individuals spend their entire life cycle within a single estuary and so factors that influence an estuary directly impact their biological characteristics. This study investigates the dietary composition of A. butcheri and the availability of their prey over two seasons in six estuaries subjected to different environmental conditions and anthropogenic influences. Gut content analysis was used to quantify diet composition and sediment samples were collected to determine prey availability in benthic community. Key environmental variables were recorded to explore relationships with diet and prey availability. A range of taxa from macrophytes, crustaceans, molluscs, teleosts and polychaetes were found ingested by A. butcheri. Bivalves were an important component to the diets in most estuaries. A reef building polychaetae had significant contribution to diets in two normally-closed systems, red algae Stylonema alsidii were dominant in one permanently-open estuary, while mussels were a main dietary component in two permanently-open estuaries. Diet also differed among seasons, with a greater variety of food items found in spring than in autumn. There was a strong relationship between prey availability (which also varied among estuaries and seasons) and the diet of A. butcheri, evidence to support A. butcheri being opportunistic feeders. This study provides insight into prey resources which support the energy requirements of A. butcheri. Paired with knowledge of benthic invertebrate compositions and environmental drivers, findings of this study can assist resource managers to better conserve key prey items which support stocks, as well as identify estuaries where A. butcheri may be at greatest risk given current and future climate change scenarios.