Taxonomic research is of fundamental importance in conservation management of threatened species, providing an understanding of species diversity on which management plans are based. The grassland earless dragon lizards (Agamidae: Tympanocryptis) of southeastern Australia have long been of conservation concern but there have been ongoing taxonomic uncertainties. We provide a comprehensive taxonomic review of this group, integrating multiple lines of evidence, including phylogeography (mtDNA), phylogenomics (SNPs), external morphology and micro X-ray CT scans. Based on these data we assign the lectotype of T. lineata to the Canberra region, restrict the distribution of T. pinguicolla to Victoria and name two new species: T. osbornei sp. nov. (Cooma) and T. mccartneyi sp. nov. (Bathurst). Our results have significant conservation implications. Of particular concern is T. pinguicolla, with the last confident sighting in 1969, raising the possibility of the first extinction of a reptile on mainland Australia. However, our results are equivocal as to whether T. pinguicolla is extant or extinct, emphasizing the immediate imperative for continued surveys to locate any remaining populations of T. pinguicolla. We also highlight the need for a full revision of conservation management plans for all the grassland earless dragons.