In this study, isotopic compositions of monthly (Global Network of Isotopes in Precipitation), event, and intraevent rain samples are used to examine the relationship between precipitation deuterium excess, the type of synoptic weather systems, and associated moisture directions in a coastal area of South Australia. The results indicate that both synoptic weather systems and associated atmospheric moisture sources influence deuterium excess values in precipitation. Rain events caused by frontal systems tend to have moisture sources from the Indian Ocean to the south of Australia. They usually have deuterium excess values of 15‰ to 25‰, depending on the moisture source direction. Rain events caused by synoptic low-pressure and trough systems tend to have inland moisture sources, and have a deuterium excess of 10‰ to 15‰. In addition to weather systems and associated moisture sources, subcloud processes alter the deuterium excess in the resulting precipitation, which is an effect that is more significant during summer when it is warm and dry. Together, these factors contribute to the seasonal variability of deuterium excess in the study area. Deuterium excess of winter frontal precipitation, resulting from minimal subcloud evaporation, is useful to infer the moisture source direction. In other seasons, deuterium excess in precipitation is more likely altered by subcloud evaporation. Nevertheless, intraevent samples in the middle of a frontal event that has experienced minimal subcloud evaporation are useful to estimate cloud deuterium excess. The results also suggest that an abrupt change in dominant precipitation weather patterns occurs between January and February, characterized by a sudden decrease in δ18O and deuterium excess.