The present-day Australian monsoon delivers substantial moisture to the northern regions of a predominantly arid continent. However, the pre-Quaternary history of the Australian monsoon is poorly constrained due to sparse and often poorly dated paleoclimate proxy evidence. Sedimentological and paleontological data suggest that warm, humid, and seasonal environments prevailed in central and north Australia during the Miocene, though it is unclear whether these were products of the Australian monsoon. We perform a series of sensitivity experiments using an atmospheric general circulation model, combined with an offline equilibrium vegetation model, to quantitatively constrain the areal extent of the Miocene monsoon. Our results suggest a weaker than modern monsoon climate during the Miocene. This result is insensitive to atmospheric CO2, although somewhat sensitive to vegetation interactions and the presumed distribution of inland water bodies. None of our Miocene experiments exhibit precipitation rates greater than modern over north Australia, in disagreement with paleoclimate record interpretations. Vegetation modeling indicates that inferred precipitation values from fossil flora and fauna could only support Miocene vegetation patterns if atmospheric CO2 was twice the modern concentration. This suggests that elevated CO2 was critical for sustaining Miocene vegetation.