The Australian Monsoonal Tropics (AMT) are one of the largest unbroken areas of savannah woodland in the world. The history of the biota of this region is poorly understood; however, data from fossil deposits indicate that the climate was more mesic in the past, and that biodiversity has been shaped by attenuation and turnover as arid conditions expanded and intensified through the Miocene and Plio-Pleistocene. The giant cave and tree geckos (Pseudothecadactylus) are distributed across three disjunct regions of relatively high rainfall in the AMT (the north-west Kimberley, the ‘Top End’, and Cape York). We present an analysis of the diversity and biogeography of this genus based on mitochondrial (ND2) and nuclear (RAG-1) loci. These data indicate that the three widely allopatric lineages of Pseudothecadactylus diverged around the mid-Miocene, a novel pattern of relatively long-term persistence that has not previously been documented within the AMT. Two Pseudothecadactylus species endemic to sandstone scarps in the west Kimberley Region and ‘Top End’ also include divergent mitochondrial lineages, indicative of deep intraspecific coalescence times within these regions. Pseudothecadactylus is a highly relictual lineage with an extant distribution that has been shaped by a history of attenuation, isolation and persistence in the face of increasingly arid conditions. The low ecological and morphological diversity of Pseudothecadactylus also contrasts with its diverse sister lineage of geckos in New Caledonia, further underlining the relictual nature of standing diversity in the former.