Previous studies have suggested that the observed winter rainfall reduction since the late 1960s over southwest Western Australia (SWWA) is consistent with what is expected from greenhouse forcing but the relative importance of potential causes is not conclusive. Here, we investigate the possibility of the rainfall reduction being a part of multidecadal variability using outputs of the CSIRO Mark 3 climate model. We find that multidecadal-long drying trends comparable to the observed exist in an experiment without climate change forcing. The model multidecadal-long rainfall decline manifests as a reduction in high-intensity rainfall events and is accompanied by an upward trend of the southern annular mode (SAM) with an increasing midlatitude mean sea level pressure (MSLP). Thus, multidecadal variability could primarily be responsible for the observed winter rainfall reduction, and could potentially superimpose on a greenhouse-induced drying trend to generate an even greater reduction than what has been observed thus far.