The Precautionary Principle is incorporated in law and policy via Ecologically Sustainable Development provisions but it is widely misunderstood and frequently ignored. Threatened species typically satisfy key elements of the Precautionary Principle in that serious or irreversible damage would be caused by inappropriate development or management, and scientific uncertainty usually surrounds an understanding of their ecology and prediction of impacts. Where could the New South Wales Threatened Species Conservation Act 1995 be precautionary? The listing process could be, but a significant amount of information is needed before a population, species or community can be listed, so nominations of taxa for which little information is available are likely to be unsuccessful. A precautionary approach to listing would be to use the IUCN classification “Data Deficient” as a category, especially if was designed to trigger a process of ecological study to gather relevant data, and a Species Impact Statement in cases where a data deficient taxon was likely to be affected by a specific development proposal. The assessment of likelihood that significant impacts will result from a development could be precautionary if determining authorities assumed a significant impact wherever scientific uncertainty existed. Currently, the application of precaution here is variable. The ‘Eight-part-test’ is effectively a checklist that is applied to determine whether there is ‘likely to be a significant impact’, thus triggering a Species Impact Statement, under the NSW threatened species legislation. Eight-part-tests that conclude no significant impact, perhaps with mitigation works, are often accepted despite questions about their validity. A conclusion that there is likely to be a significant impact will not necessarily halt a development proposal, because compromises with socio-economic benefits can be made. We urge a more thorough application of the Precautionary Principle, to force decision-makers to acknowledge this compromise, rather than hiding behind the pretence that lack of knowledge implies no detrimental impact.