Jurassic sedimentary successions in eastern Australia are widely thought to have been deposited in nonmarine environments. Thus, the discovery of low-diversity dinoflagellate cyst assemblages with associated colonial algae in the Walloon Coal Measures of the western Surat Basin provides new evidence of either a short-lived marine transgression or the very rare occurrence of nonmarine dinoflagellate cysts in pre-Cretaceous strata. Their small size, thin walls and simple proximate shapes are typical of freshwater to brackish dinoflagellate cysts, as are the low species richness and high dominance nature of the assemblages. Two new species of dinoflagellate cysts (Moorodinium crispa sp. nov. and Skuadinium fusum sp. nov.) and a new species of colonial algae (Palambages pariunta sp. nov.) are described from these assemblages. Tidal channel and tidal mudflat facies associated with these assemblages provide evidence of a possible upper estuarine setting. Support for a marine incursion is provided by U–Pb dating. This yielded an age of 150.11 ± 0.04 Ma (∼100 m above the dinoflagellate cyst assemblage in the Indy 3 well) that ties to an episode of high eustatic sea level during the Tithonian. Thus, a marine transgressive event during the Tithonian may have allowed dinoflagellates to migrate into the interior of the Australian continent. If these dinoflagellate cysts are found more widely, rather than being just an isolated occurrence in this well, they may provide a useful correlative tool for tracing distinctive brackish to marginal marine flooding surfaces in continental successions in eastern Australia.