The first paper dealing with revision of the Australian members of cosmopolitan genus Diomus Mulsant treats 30 species, mostly with distinct colour pattern, that can be identified using external characters. Nomenclatural history, diagnoses, illustrations and distribution are provided for each species. Fifteen new species are described: Diomus ancorus sp. nov., D. bimaculatus sp. nov., D. brookfieldi sp. nov., D. capital sp. nov., D. kuranda sp. nov., D. kosciuszko sp. nov., D. millaamillaa sp. nov., D. marmorosus sp. nov., D. ningning sp. nov., D. reidi sp. nov., D. storeyi sp. nov., D. tinaroo sp. nov., D. torres sp. nov., D. weiri sp. nov., D. zborowskii sp. nov.. The following new synonyms are proposed: Diomus sphragitis (Weise, 1885) = Scymnus trilobus Lea, 1902 = Scymnus indistinctus Lea, 1902; Diomus sydneyensis (Blackburn, 1892) = Scymnus mimicus Lea, 1902 = Scymnus pectoralis Lea, 1902 (new synonyms). Scymnus australis Blackburn, 1889 is transferred to Diomus (new combination). Lectotypes are designated for the following taxa: Diomus rusticus Weise, 1859; Scymnus australis Blackburn, 1889; S. corticalis Lea, 1908; S. cucullifer Blackburn, 1892; S. ementitor Blackburn, 1895; S. flavifrons Blackburn, 1889; S. flavolaterus Lea, 1926; S. indistinctus Lea, 1902; S. jocosus Blackburn, 1892; S. kamerungensis Blackburn, 1895; S. meyricki Blackburn, 1889; S. mimicus Lea, 1902; S. notescens Blackburn, 1889; S. pectoralis Lea, 1902; S. (D.) pumilio Weise, 1885; S. (D.) scapularis Weise, 1885; S. striatus Lea, 1902; S. sydneyensis Blackburn, 1892; S. tenebricosus Boheman, 1859 and S. trilobus Lea, 1902.