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Octocorals have been recognised as abundant and ecologically significant on many deep-sea features. However their taxonomy and distribution remains poorly understood due to inadequate historical literature and a paucity of definitive morphological characteristics, and this is now hindering conservation and management in many of our important deep-sea marine ecosystems. For example, a decadal study of fishing impacts on seamounts south of Tasmania, Australia revealed a great diversity of octocorals, including two commonly collected and widely distributed genera, Anthothela and Primnoisis, but specimens could not be identified to species due to taxonomic confusion within the groups and hence could not be considered in diversity assessments and conservation measures. The taxonomy of these genera is revised herein in order to prescribe genus and species level morphological definitions, phylogeny and geographical extent. A multi-disciplinary approach was used combining morphological characteristics such as colonial branching patterns, polyp structure and sclerite form and arrangement, and phylogenetic reconstructions using two mitochondrial gene regions (mtMutS and igr1‚Äö-COI). Anthothela (Family Anthothelidae), with six nominal species globally, is here divided into four genera, two of which are new. Anthothela is found to have three valid species, A. grandiflora, A. pacifica and A. tropicalis, another species Spongioderma (?) vickersi is reassigned to Anthothela and two new species A. aldersladei and A. quattriniae, are described. Anthothela argentea and A. nuttingi are reassigned to Victorgorgia (Family Anthothelidae) and two new species of this genus, V. eminens and V. nyahae are described. These are the first records of Anthothela and Victorgorgia from Australia. One new genus, Williamsius, is described for A. parviflora, which is restricted to South African waters, and Lateothela anitorkilda n. gen., n. sp. is described to incorporate north Atlantic Ocean specimens which have been traditionally been mistaken for A. grandiflora. There was good congruence between morphological characteristics and molecular data at a generic level, but at the species-level morphological and genetic variation was very low. Anthothela and Lateothela n. gen. are found to be closely related to some nominal Alcyonium species and the Family Anthothelidae is shown to be paraphyletic. The genus Primnoisis (Isididae) is retained with 7 of the 8 nominal species, P. antarctica, P. rigida, P. ambigua, P. delicatula, P. fragilis, P. formosa, P. mimas, validated and the eighth, P. sparsa is synonymised with P. antarctica. In addition, the species Mopsea gracilis is reassigned to Primnoisis and five new species are described; P. chatham, P. erymna, P. millerae, P. niwa and P. tasmani. Most of the species fell into two clear groups, defined both by morphology and genetics, for which two new sub-genera are proposed (P. (Primnoisis) and (P. Delicatisis)). Three species, P. ambigua, P. mimas and P. tasmani n. sp., could not be placed reliably in either sub-genus due to distinctive morphological features or genetic dissimilarity. It was not possible to confirm the monophyly of the genus due to unresolved relationships with the closely related genus Notisis and an undescribed Mopseinae genus. P. tasmani n. sp., with a distribution restricted to the southeast of Australia, is positioned basal to all other Primnoisis species based on DNA sequence data, suggesting southern Australia as the origin of the genus, with subsequent vicariant speciation after the separation of Gondwana and initiation of the Antarctic Circumpolar Current (ACC). North of the ACC, species are restricted to general geographical features such as seamount complexes, ridges and plateaus. Three species recorded south of the ACC are found to have extensive distributions around the Antarctic continental shelf and two further species appear to be widely distributed. P. millerae n. sp. is found to be separated by depth from other species on the Antarctic continental shelf and P. fragilis appears to have limited connectivity between vastly separated Antarctic populations, although low variability in the gene regions used and small sample size prohibit definitive conclusions. These results illustrate significant undescribed diversity in the octocorals of the Southern Ocean and indicate that without comprehensive taxonomic reviews, current biodiversity estimates are likely to be grossly inaccurate, even at a genus level. This research will facilitate future ecological and conservation research on these octocorals by allowing more robust identification, and producing accurate geographic distributions and connectivity assessments. These in turn will guide conservation efforts to protect these poorly understood deep-sea communities.