Cotter Reservoir was enlarged from 4 to 76 GL to improve regional water security. This study investigates the long-term impacts of the enlargement of Cotter Reservoir, Australian Capital Territory on the resident fish community. We examined, fish community dynamics across three phases of reservoir enlargement: before filling, during filling, and operational. A comprehensive monitoring program focused on the responses of native species, particularly the endangered Macquarie perch (Macquaria australasica), as well as non-native fish such as rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta), and goldfish (Carassius auratus). Filling and early operation phase saw a dramatic shift in fish community composition, largely driven by high captures of adult Macquarie perch and goldfish. Proliferation of goldfish likely contributed to the establishment of a breeding colony of piscivorous birds on the reservoir. Reservoir enlargement also facilitated the rise and fall of the adult brown trout population in the reservoir. Latter operational phase saw the fish community composition reach equilibrium and return to a composition similar to before filling. The study underscores the importance of long-term ecological monitoring in quantifying impacts of large-scale water infrastructure projects fish communities.