Risk analysis is a management tool that is becoming increasingly common in biosecurity because it aids decision-making in the face ofuncertainty. A risk analysis model [referred to as an organism impact assessment (OIA)] is described, one that was developed in New Zealand to facilitate the management of incursions of introduced aquatic species in a post-border (after quarantine is breached) scenario. The New Zealand biosecurity risk-management framework assesses ecological, cultural, social, and economic values congruently, ensuring that a transparent and objective framework is established with clearly stated ecological and socio-political imperatives. Using expert heuristics and published and observed data, the present study assesses the likelihood that a target introduced species will have ecological, cultural, social, and economic impacts. The consequences (impact and/or change) of such events are then determined, to establish a relative risk ranking, using consequence matrices to aid assessment of the ecological, cultural, social, and economic value impacts of species unintentionally introduced to New Zealand. To illustrate the risk model, the OIA for the incursion of the fresh-water diatom Didymosphenia geminata is presented. The likelihood and consequences resulting in risk pertaining to the introduction of D. geminata varied across regions, but based on public perception at the initial incursion location, Southland, D. geminata was considered to be an extreme risk across all core values.