A roaded catchment (RC) is a representative type of artificial catchment for rainwater harvesting. The rainfall-runoff threshold value of the RC is the main factor which influences the system efficiency and cost. Antecedent soil moisture condition is an important factor which impacts on the determination of the rainfall-runoff threshold value. In this study, rainfall-antecedent soil moisture condition-runoff relationships and the potential efficiency of RCs are presented. Rainfall and runoff data monitored at research sites in Merredin and Mount Barker are used to determine this relationship. Two antecedent moisture criteria; Antecedent Moisture Conditions (AMC) and Average Antecedent Precipitation (AAP) are used to analyse the relationship between previous rainfall and soil moisture for each RC. Monitored results show that AMC is not that suitable to show the relationship between rainfall and antecedent soil moisture condition of the RC in the dryland of Western Australia and it is recommended to use AAP to determine this relationship.