<p>Certain macrophytes can release allelopathic compounds that inhibit nuisance cyanobacterial growth. To determine the allelopathic potential of submerged macrophytes species native to southeastern Australia, plant filtrate laboratory bioassays were performed against a toxic strain of <i>Microcystis aeruginosa</i> under nutrient-enriched conditions. Macrophytes were collected from Prospect Reservoir, located in Sydney, Australia, during three austral seasons, and plant material was incubated under light and dark conditions. Seven submerged macrophyte species were screened and antialgal effects were identified through the comparison of final cell density and lag phase duration to a control. Growth inhibition was identified in all species, including, for the first time, <i>Vallisneria australis</i> and <i>Potamogeton octandrus</i>, both from genera previously considered allelopathically weak. Filtrates from summer-collected plants and leachate from decomposing plants (dark-incubated) promoted growth inhibition. A higher total phenolic content (TPC) was measured in dark-incubated plant filtrates, and a weak positive relationship between TPC and lag phase duration was observed. A high variability in inhibitive effects was observed indicating variable allelochemical production within individual plants. These findings suggest that allelochemical release may be more widespread among submerged macrophytes than previously assumed.</p>