The eyeless and obligate cave-dwelling genus Tartarus Gray, 1973 consists of a group of rare and highly specialised species of spider. The genus is only known from caves on the Nullarbor Plain, spanning South Australia and Western Australia, and each species is currently only known from single caves. This extreme short-range endemism, in combination with documented threats to the species, puts them at high conservation-significance. We describe two species, Tartarus subtilis sp. nov., the first species of Tartarus recorded from South Australia, and Tartarus evanescens sp. nov. from Western Australia. We present the first sequence data for this genus of spiders and use this to support morphological diagnoses. We also describe the male of Tartarus thampannensis Gray, 1992, which was previously only known from a juvenile female. We apply International Union for the Conservation of Nature frameworks to conduct conservation summaries for each species, finding that all species treated in this manuscript are of conservation significance, and we discuss threats to the species and cave systems. These findings provide new information on the diversity of obligate cave-dwelling species in the caves of the Nullarbor, and on the importance of the region as a centre for biodiversity and endemism.