This paper provides the first monographic account of south-eastern Australian representatives of the Hapalidiaceae (Corallinales, Rhodophyta). The Hapalidiaceae includes those Corallinales whose tetrasporangia produce zonately arranged spores, and whose tetrasporangia/bisporangia are borne in conceptacles, produce apical plugs, develop beneath multiporate plates or roofs, and are not enclosed individually within calcified sporangial compartments. The Hapalidiaceae contains the subfamilies Choreonematoideae, Melobesioideae and Austrolithoideae, all formerly placed in the Corallinaceae sensu lato. The Choreonematoideae is represented in south-eastern Australia by a single species, Choreonema thuretii. The Melobesioideae is represented by five genera (Melobesia, Lithothamnion, Phymatolithon, Mesophyllum, Synarthrophyton) and eight species. Synarthrophyton pseudosorus sp. nov. is newly described. The presence of tetrasporangial conceptacles occurring in irregularly shaped dense clusters that contain both discrete conceptacles and fused groups of conceptacles delimits this species from all other species of Synarthrophyton. Morphological and anatomical accounts are provided, including keys, information on distribution, habitat and nomenclature. Brief biogeographic comparisons between south-eastern Australia and neighbouring regions are also made.