Backobourkia, a new Australasian genus of orb-weaving spider (family Araneidae Clerck, 1758) is proposed with Backobourkia heroine (L. Koch, 1871) as type species. Two other species are included in the genus: B. brounii (Urquhart, 1885) comb. nov. (here removed from synonymy with B. heroine) and B. collina (Keyserling, 1886) comb. nov. (= Araneus reversus Hogg, 1914, new synonymy). A phylogenetic analysis places Backobourkia within the 'coxal hook clade' of the subfamily Araneinae Clerck, 1758 and close to Eriophora Simon, 1864; however, Backobourkia differs from other Australasian Araneinae by the proposed synapopomorphy of a long fl ange at the base of the median apophysis in males. Molecular phylogenetic analysis of a fragment of the mitochondrial gene cytochrome c oxidase subunit 1 (COI) supports the monophyly of Backobourkia in a limited dataset that includes exemplars of Australasian Araneidae and Eriophora ravilla (C.L. Koch, 1844) (type species of Eriophora Simon, 1864). Backobourkia brounii and B. heroine are sister taxa in this analysis. Backobourkia collina is unusual within the genus and amongst most Araneinae as it displays extreme sexual size dimorphism (SSD), with females on average more than four times the size of males. Comparison with the other two Backobourkia species (and other Araneinae) suggests male dwarfi sm rather than female gigantism for this SSD. Some morphological differences of B. collina males to its congeners are evident, possibly due to its reduced size. These include characters of putative phylogenetic signal at the genus level such as eye position, shape of male pedipalp sclerites and the absence or presence of endite teeth or coxal hooks. Backobourkia species are common throughout Australia, with B. heroine more prevalent in the western and B. brounii in the eastern parts of the country. Backobourkia collina is mainly found throughout the arid regions. Backobourkia brounii is also found in New Zealand and B. heroine in New Caledonia. Araneus felinus (Butler, 1876), originally described as 'Allied to E. heroine', is here considered a nomen dubium as the type specimen appears to be lost and an accurate identifi cation based on the original description is not possible. The New Zealand Eriophora decorosa (Urquhart, 1894) is transferred to Acroaspis Karsch, 1878, A. decorosa (Urquhart, 1894) comb. nov.