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Using the Great Fish River, South Africa, Laurenson and Hocutt (1986) proposed a colonisation theory outlining the characteristics of successful invaders and the properties of aquatic habitats that facilitate successful fish invasions. The Great Fish River has modified flow due to an inter-basin water transfer (IBWT). The present study evaluated the current state of invasions and probable mechanisms facilitating establishment of non-native fishes within this river. We used historical literature on the non-native fishes and associated this information with the recent distribution records of these fishes within this system. We hypothesised that non-native fishes would reflect life-history traits and environmental associations that were consistent with adaptability to flow-modified disturbance. Of the 11 non-native fish species recorded within this system, seven have established, three have failed to establish and one is uncertain. The Orange–Fish IBWT and angling were the main vectors of invasions, accounting for 36% and 46% of the non-native species, respectively. Life-history traits, such as large size, greater longevity and wide habitat tolerance, appear to have facilitated establishment of most species. Our findings indicate that both intrinsic and extrinsic factors may explain the invasions of non-native fishes within the Great Fish River and possibly other similarly disturbed systems.