Two acritarch assemblages from the Early Ordovician Tremadocian and Arenigian stages are recognised from the subsurface Innamincka Formation (previously Innamincka Red Beds). The Tremadocian assemblage probably ranges back to the Late Cambrian, but the Arenigian assemblage is dominated by abundant, distinct Early Ordovician species of world-wide distribution. Acritarchs are found to be useful in regional correlation by dating sediments in those wells lacking macroscopic fossil evidence. The studied acritarch assemblages are environmentally controlled. Open marine deposits contain generally diverse and morphologically complex plankters; assemblages from the upper subtidal delta-influenced silty mudstone are dominated by simple spheroids and contain spinose plankters and filamentous cyanobacteria; delta-influenced intertidal mudstone contains exclusively cyanobacterial filaments and spheroidal acritarchs; pro-delta mudstone contains mainly spinose and spheroidal acritarchs. Acritarch assemblages from open marine and pro-delta deposits are useful for intercontinental stratigraphic correlation.