The National Carp Control Plan (NCCP) is developing a plan for the potential release of the carp virus to control invasive common carp, Cyprinus carpio, in Australian freshwater environments. The NCCP commissioned a program of scientific, social, and economic research to develop the knowledge required to enable an informed decision of whether the virus release should proceed. Part of the scientific program involved an exploration of the medium to long-term ecological effects likely to result from major carp population reductions (i.e. over a 10 to >20-year timescale). To assess the likely ecological changes, we used both an expert elicitation process and an evaluation of the scientific evidence. In addition to predicting the ecological effects of carp reductions under various biomass scenarios (no control, 25%, 75%, and 100% reductions) the project includes an assessment of the confidence in predictions and a systematic review of scientific evidence for prioritised causal pathways. The outcomes of this project will inform management of likely positive and negative ecological outcomes of viral release under different scenarios and inform other NCCP projects that are evaluating costs and benefits and assessing community attitudes to carp biocontrol.