The southern Australian Helminthocladiaceae, comprising Nemalion (one species), Helminthora (two species, including H. lindaueri from New Zealand), Helminthocladia (four species, with H. beaugleholei sp. nov. and H. dotyi sp. nov.), and Liagora (four species, with L. codii sp. nov.) are described in detail. The generic limits of Helminthora and Helminthocladia are clarified, Nemalion multifidum is considered a synonym of N. elminthoides, Helminthocladia australis is considered to include the South African H. papenfussii and the Californian H. californica, and notes are given on two Western Australian species of Liagora, L. australasica and L. preissii.