Australian lakes, especially reservoirs with protected catchments, provide an ideal refuge for a vast array of microalgae such as desmids. The use of desmid assemblage data for water quality monitoring and habitat assessment has been successfully applied in Europe and its uptake is increasing in other regions. A case study was developed to determine the trophic status of reservoirs via historical records of desmid phytoplankton data from six water storages southwest of Sydney, New South Wales, Australia. All six reservoirs are situated in narrow rocky basins atop of sandstone, west and/or southwest of Sydney, Australia. The surrounding catchment of these reservoirs at the time of this study was predominantly protected by eucalypt forest, with access restricted to the public. However, in the case of Lake Burragorang, several different rivers within its basin have undergone intense land use activities such as grazing from livestock, as well as receiving treated sewage effluent from two nearby towns. Plankton desmid assemblages from a 1973 study by Thomasson were collated and scored for individual species ecological ratings (trophic status). Data-deficient species from the previously mentioned floras were cross-checked and updated as required, and each species was then tallied for each category and plotted for analysis. Avon, Cataract, Cordeaux and Woronora reservoirs were dominated by species rated as commonly found in meso-oligotrophic waters. Subdominant proportions for these sites were also composed of strictly oligophillic and oligo-eutrophillic species; with only one desmid classified as meso-eutrophillic, detected in Lake Cataract. In contrast, Lakes Burragorang and Prospect were dominated by species having broader ecological tolerances, i.e., oligo-eutrophillic. This trend was especially pronounced at Burragorang where the remaining subcomponent assemblages of species found were from habitats rated as mesotrophic, meso-eutrophic and eutrophic. This was less noticeable in Prospect as that site also included oligophillic, meso-oligotrophillic and meso-eutrophillic desmids. This study highlights the utility and potential for desmid phytoplankton data sets to assist limnologists in assessing the trophic status of reservoirs in a historical context where other data may be inconclusive or lacking, e.g., nutrient levels. Previous studies of Lake Burragorang's characteristics and physicochemical properties during 1961 - 1980 could not conclusively determine a trophic upsurge or trend towards eutrophication. The desmid assemblage data set presented here clearly indicates that during the study period of February to October 1964, Lake Burrgorang was markedly different to the other nearby reservoirs in desmid species composition, being dominated by species with a broader trophic tolerance that are commonly observed in habitats undergoing eutrophication. These results are consistent with the historical account of the characteristics of the catchment area and land usage practices at the time of when these studies were conducted.