The development of a model that predicts the levels of microorganisms in urban stormwater will aid in the assessment of health risks when using stormwater for both recreational uses and as an alternate water resource. However, the development of such a model requires an understanding of the dominant processes that influence the behaviour of microorganisms in urban systems. Using simple and multiple regression analyses this paper determines the dominant processes which affect the inter-event variability of the microbial indicator Escherichia coli (E. coli ) in four urbanised catchments. The results reveal that a number of antecedent climatic conditions, together with rainfall intensity, can significantly explain the inter-event variation in wet weather E. coli levels.