AbstractIn the austral summer, parts of southeastern Australia are prone to heavy rainfall that causes major riverine flooding and fatalities. Easterly flow associated with an anticyclone in the Tasman Sea, large moisture transports from the Coral Sea, and upper tropospheric cyclonic disturbances all contribute to these heavy rainfall episodes. However, questions regarding their synoptic dynamics remain, including which of these ingredients are the most critical. These questions are addressed by comparing composite pressure and moisture fields of heavy rainfall days over selected regions with non‐heavy rainfall days that have a similar synoptic pattern. A synoptic climatology is constructed for this purpose by ‐means cluster analysis of 500 hPa geopotential height anomalies from the European Centre for Medium‐range Weather Forecasts Reanalysis v5, for all December to March days over a period of 40 years. Heavy rainfall days in the wettest clusters have negative 500 hPa geopotential height anomalies immediately west of the affected region that are stronger on average than those of non‐heavy rainfall days. Their accompanying distributions of surface pressure, precipitable water, and vertical motion are consistent with cyclonic baroclinic development and are preceded by anticyclonic Rossby wave breaking. Heavy rainfall days also show an increased frequency of blocking near E; however, this peaks 1–2 days after the onset of heavy rain. Regional rainfall in these clusters shows strong sensitivity to lower pressure immediately westward but little sensitivity to high pressure in the Tasman Sea until after the commencement of rain. A companion study using the same cluster analysis illustrated the link between anticyclonic Rossby wave breaking and heatwaves in southeastern Australia. These latest results highlight the upper cyclonic anomalies that often form on the equatorward flank of anticyclonic Rossby wave breaking as the key ingredient separating days with a favourable synoptic‐scale pattern of surface high pressure into those that rain heavily and those that do not.