Many freshwater fishes have been introduced outside their natural range. The consequences have included the decline or extinction of native fishes, principally due to competition and predation. Redfin perch (Perca fluviatilis) is a highly efficient predatory fish species that was introduced to Australia in the 1800s. It now has a broad distribution in the Murray–Darling Basin, but its impacts on native fishes are largely unstudied. It often cohabits with native golden perch (Macquaria ambigua ambigua), which is similar from a trophic ecomorphology perspective. We examine prey selection and diet overlap of adult redfin perch and golden perch under contrasting hydrological conditions in terminating lakes of the Murray–Darling Basin. Prey selection by both species varied substantially between drought and flood conditions. Diet overlap of redfin perch and golden perch was significant only during flood, and was apparently related to pelagic prey availability. There were dietary differences during drought that imply that resource partitioning occurred between the perches, possibly because competitive interactions were intensified. Conversely, the promotion of pelagic prey fishes during flooding apparently facilitated resource sharing. The findings suggest that redfin perch can directly compete with native piscivores for prey. The potential impacts on native piscivores and small-bodied fish populations warrant further experimental and field investigations.