The allelopathic effects of three exotic macrophytes, Elodea canadensis, Cabomba caroliniana and Egeria densa, and three native species, Potamogeton crispus, Potamogeton ochreatus and Ceratophyllum demersum, on two species of cyanobacteria, Anabaena variabilis and Synechococcus sp. and a green algal species, Chlorella sp. were tested under laboratory conditions. Experiments were conducted using live material. Allelopathy was considered to have occurred if the growth of the cyanobacterial or algal species was significantly lower in treatments comprising live macrophyte material in comparison to controls (without live material). Both C. caroliniana and E. canadensis exerted significant negative effects on the growth of the target organisms, with the strongest effect exerted by C. caroliniana. The allelopathic effects of the invasive species were higher than those of native macrophytes (except C. demersum). These findings suggest that allelopathy may have an important role in facilitating the success of exotic macrophytes in temperate Australian wetlands.