Rapid climatic change at the beginning of the Oligocene epoch is concordant with global biotic turnover in the fossil record. However, while Southern Hemisphere geological movement played a key role in shaping these global climatic shifts, given generally poor terrestrial fossil records, evidence for matching turnover in entire Austral biotas is lacking. Emerging comprehensive phylogenetic frameworks provide alternative avenues to explore for signals of mass turnover or restructuring. Here, we combine phylogenetic data with empirical and simulation-based approaches to understand the temporal dynamics of the origins of a diverse and highly endemic continental biota (Australian lizards and snakes). These analyses indicate that the temporal clustering of major radiation ages in Gondwanan endemic lineages and immigration into Australia is narrower than expected under time-continuous models assuming no overarching external perturbation. Independent phylogenetic dating analyses further indicate that the timing of both processes is concentrated in the period post-dating the Eocene–Oligocene transition (~34 million years ago). Epoch-defining processes around the start of the Oligocene appear to have also played a decisive role in reshaping a diverse Southern Hemisphere biota—by both re-setting Gondwanan endemic diversity and opening the way to successful immigration from the north.