A new species of Opiinae, Diachasma dentatum Shirley, Restuccia & Ly, is described from Australia. This species is similar to several other Australian opiines previously described or included in the genus Diachasma, but the mandibles are unusually broad, nearly exodont. Notable differences between Australian and Palaearctic Diachasma are discussed. Diachasma tasmaniae Fischer, 1995, originally described from Tasmania and New South Wales, is newly recorded from Victoria. Diachasma rufipes Szépligeti, 1905 is transferred to Notiopambolus, new combination.
Keywords: Atoreuteus; Lord Howe Island; New Guinea; Notiopambolus.