Accidental and deliberate translocation of native fish outside their endemic range occur all too frequently in Australian freshwater ecosystems, with more often than not, detrimental outcomes. The barred grunter (Amniataba percoides) is a small-bodied teraponid fish that is found naturally throughout the freshwater streams and rivers of tropical and subtropical Australia. Through translocation, the species is also now found in several rivers and impoundments well outside its natural range, including in the Clarence River system, north-eastern NSW. We sampled barred grunter monthly within the Clarence River system over an 18-month period to determine their distribution, age, growth, reproduction and diet. Individuals were captured regularly throughout the freshwater reaches of the lower Clarence system, ranging from 76–197 mm fork length, 7–123 g in weight and we aged them from 0+ to 6+ years. The population structure and gonadosomatic index (GSI) values were indicative of annual spawning and recruitment since their introduction. The contents of stomachs revealed an omnivorous diet dominated by aquatic plants and to a lesser degree, micro- and macro-crustaceans. Our study has confirmed that the introduced barred grunter has successfully established in the Clarence River system. We discuss the potential future impact of barred grunter on the Clarence system, and suggest possible management options for this and other incursions of translocated fishes in Australian rivers.