Rainfall over Victoria during the cool season (April to October) has been unusually low since the beginning of the Millennium Drought in 1997 (~12% below the 20^th century average). Cool season rainfall contributes two-third to annual rainfall and is very important for many crops and for replenishing reservoirs across the state. In this study, we use 24 CMIP5 climate models to estimate (i) the extent to which this drying is driven by anthropogenic forcing, and (ii) future rainfall, taking both external forcing and internal natural climate variability into account. All models have preindustrial, historical, and 21st century (RCP2.6, RCP4.5 and RCP8.5) simulations. We found that rainfall in the past two decades is below the preindustrial average in two-thirds or more of model simulations. Given that natural, internal variability might dominate the direction of rainfall change for a 20 year period, this agreement amongst the models on the sign of change suggests that it is a forced response. However, the magnitude of the multi-model median externally-forced drying is equivalent to only 20% of the observed drying, suggesting that the drying is dominated by internally-generated rainfall variability, or that the models are not fully representing all the processes that contribute to the observed rainfall decline. The forced signal, however, rapidly emerges in an estimate of the current climate centred on 2020 (2010 – 2029), with drying evident in over 90% of the model simulations. Looking further forward (2018 – 2037) period, model results suggest that there is only a ~12% chance that internal rainfall variability could completely offset the externally-forced drying. However, by the late 21^st century the externally-forced drying under RCP8.5 is so large that internal variability appears too small to be able to offset it.