Identification of threatened species in the marine environment comes with significant unique challenges, including high access costs and difficulties collecting population trend and distribution data. The IUCN Red List of Threatened Species plays a pivotal role in monitoring biodiversity and informing conservation actions, but, for marine species, includes large taxonomic gaps and is infrequently updated. These issues reduce the potential value of the Red List for guiding management actions, inflating risk of extinction. To assess the comprehensiveness and adequacy of Red List assessments for marine species, we quantified population trends for 638 common shallow-reef species around continental Australia (2008 – 2022) using time series data from national standardised reef monitoring programs. A total of 139 species showed significant population trends, including 78 declines. Most (47) declining species are endemic to Australia, of which 14 appear inaccurately listed as “Least Concern”, and the oldest of these Red List assessments two decades outdated. A strong taxonomic bias exists with 29 of 47 declining endemics comprising temperate macroalgae and invertebrate species, including 28 species still to be formally evaluated. To avoid unnoticed extinctions, timely assessments and adaptive management strategies are required, possible only through extensive monitoring and reporting programs covering many taxa.