The millennium drought strained water resources in south-eastern Australia. To increase domestic water security in the Australian Capital Territory, Cotter Dam was enlarged 20-fold. Alien fish populations in the Cotter Reservoir were monitored before and during filling, largely to determine if changes in their population size and structure due to reservoir enlargement had resulted in detrimental impacts on populations of threatened Macquarie perch and two-spined blackfish. Since filling, populations of goldfish have increased, likely in response to a surge in available resources as the reservoir fills. The increase in goldfish is likely to be a driver in the establishment of a breeding colony of cormorants (never before observed in the Cotter Catchment). Size and abundance of rainbow trout has not changed since filling began, though there has been a marked increase in brown trout abundance. This is of some concern as a steep increase in an apex predator such as brown trout could increase trout predation upon the Macquarie perch population. Eastern gambusia and oriental weatherloach remain rare captures with current capture techniques; however, ad hoc observations of both of these species suggest they are common throughout the reservoir. Since filling began in 2013, there has not been any link between alien fish population change due to reservoir filling and increase in detrimental impacts to threatened fish populations in the Cotter catchment.