Australia has a long history of success with biocontrol, but some notorious failures. We review the benefits, effectiveness and risks of cyprinid-herpesvirus-3 to target common carp (Cyprinus carpio). Success could boost native fish biomass to more than double the current level, but there is little evidence demonstrating that release can deliver effective, let alone safe, biocontrol of carp in Australian waterways. The virus may already be present in Australia. Overseas it has caused intermittent, localized mass-mortalities in aquaculture ponds and lakes, typically leading to short-term water quality problems and then recovery of carp populations. Experiments on Australian biota have reported unexplained high mortality rates of recreationally important and threatened fishes. As matters of national and international environmental significance under the Environment, Protection and Biodiversity Conservation Act 1999, we strongly encourage the Australian government to re-evaluate the susceptibility of all threatened native species and field-test the effects on free-living common carp in wetlands, rivers or mesocosms, including food webs connected to fish and waterbirds. Despite political pressures, there is no environmental justification to rush this viral biocontrol release. Before large-scale releases, contained field trials and independent species-specificity testing are essential, coupled with transparent peer-reviewed reporting. Australia cannot afford another disastrous biocontrol.