Historically, underwater forests of kelp dominated subtidal rocky reefs in temperate and subpolar regions globally. At present, these ecosystems are experiencing unprecedented levels of decline due to physical, biological, and anthropogenic factors, including accelerated climate change, large-scale oceanographic current alterations, marine species distribution shifts, and overfishing. In Australia, giant kelp forests have been classified as an ’endangered marine community type’ under the Federal Environment Protection and Biodiversity Act since 2012 due to an alarming rate of decline over the past few decades. Given the ecological and social importance of these forests as ecosystem services providers, it is necessary to understand the magnitude of change to create accurate recovery targets for these ecosystems at risk. Using admiralty charts from the 19th century, this research seeks to quantify the historical extent of kelp forest along five sites on the east coast of Tasmania, comparing past distributions with modern kelp surveys (1953, 1986, 1999, 2019), and determine areas of kelp forest persistence over the past century. Admiralty charts present an opportunity to identify the extent and distribution of coastal habitats at high-spatial resolution and help create ecosystem baselines from which to inform habitat management and restoration efforts.