The importation and use of microalgae as live feeds for aquaculture and research poses significant marine biosecurity risks as it represents a poorly or unregulated mechanism for the introduction of non-native species. In many jurisdictions, the importation of a good is controlled by an Import Health Standard (IHS); however within New Zealand, no IHS explicitly exists for microalgae and no import risk assessments have occurred. This represents an unregulated pathway that requires urgent management attention. To address this problem, an 8-step decision-tree (based on non-probabilistic if/then statements) was developed. The decision-tree applies the same set of criteria to all species and strains, ensuring that future importation of microalgae could be assessed in a consistent, robust, transparent and defensible manner. To assess the validity of the decision-tree, all New Zealand microalgae imports from 1998-2005 were assessed using the decision-tree and then compared against a derived risk assessment for the same data-set. Microalgae imports that pose a risk to New Zealand have been imported and released into the environment, indicating that this unregulated pathway needs improved biosecurity management. Both aquaculture and research agencies are responsible for releasing microalgae imports. The decision-tree was a more conservative method than a derived risk assessment process.