The Australian endemic lycaenid Pseudalmenus H.H. Druce, 1902 occupies a unique phylogenetic position within the Theclinae–Polyommatinae assemblage. Although the genus exhibits complex geographic variation, it has long been considered to be monotypic. However, evidence from adult phenotype (colour pattern), immature stages (final instar larva) and ecology (ant specificity) (total of 10 unique character states) as well as limited genetic data (mitochondrial COI) suggest that there are two species, namely, P. chlorinda (Blanchard, 1848) from Tasmania and the mainland of south-eastern Australia and P. barringtonensis Waterhouse, 1928 stat. rev., which is allopatric and narrowly restricted to montane areas in northern New South Wales. Examination of the ‘holotype’ male of P. barringtonensis in the Australian Museum showed that it is a fake, although the data label is genuine; the specimen is actually P. chlorinda chloris Waterhouse & Lyell, 1914 that has been modified with red paint to resemble P. barringtonensis. The true holotype is currently missing, but a specimen in the Australian Museum (registration No. K199026) that is part of the Colin W. Wyatt Theft Collection with a fictitious label is almost certainly the true holotype of P. barringtonensis. We discuss the history of this most unusual and bizarre circumstance and conclude that Wyatt stole the holotype sometime in 1946 before he returned to England (∼72 years ago) and fabricated the fake holotype as a replacement specimen. Such a fraudulent and unprecedented act surely ranks as Australia’s greatest taxonomic fraud.