Redfin perch (Perca fluviatilis) introduced to the Southern Hemisphere has contributed to the decline or localised extirpations of native fishes, principally due to predation. It has been widely recorded in the Murray–Darling Basin, south-eastern Australia, since the 1920s but the ecological consequences are largely undetermined. The purpose of this study was to examine the diet of juvenile redfin perch in Lake Alexandrina to assess its potential impacts on native biota in two distinct habitats (channel and lake). We proposed that the broad dietary composition of juvenile redfin perch matches that of its natural range (small decapods and insects). Most juvenile redfin perch with prey items in their guts, however, had consumed native fish. There was variability in the diet of redfin perch between the channel and lake where gudgeons and gobies, respectively, were targeted. Unexpectedly, otolith ageing revealed that the redfin perch were larger and shifted to piscivory at a much younger age compared with populations in its native range. Among other ecological issues, the findings are pertinent to threatened small-bodied fish populations in the Murray–Darling Basin. More broadly, they suggest that a generalist feeding behaviour can lead to the early onset of piscivory in alien fish populations.