Abstract. Inshore reefs located on the coastline of northern Australia are foraging grounds for several threatened species of sea turtle which feed on brown algae present on the reef flat. Using Landsat TM visible bands, an iterative digital image classification is used to delineate the major reef flat zones. In addition, TM band 4 is used to assist in the identification of algal growth, showing that this band can successfully separate substrate types up to a depth of 35cm in clear calm water. The classification results reveal that the TM data have the potential to identify crucial turtle feeding grounds along the vast coastline of northern Australia.