Adaptation of weeds to water stress could result in the broader distribution, and make weed control task increasingly difficult. Therefore, a clear understanding of the biology of weeds under water stress could assist in the development of sustainable weed management strategies. Avena fatua (wild oat) and A. ludoviciana (sterile oat) are problematic weeds in Australian winter crops. The objectives of this study were to determine the growth and reproductive behaviour of A. fatua and A. ludoviciana at different soil moisture levels [20, 40, 60, 80, and 100% water holding capacity (WHC)]. Results revealed that A. fatua did not survive and failed to produce seeds at 20 and 40% WHC. However, A. ludoviciana survived at 40% WHC and produced 54 seeds plant-1. A. fatua produced a higher number of seeds per plant than A. ludoviciana at 80 (474 vs 406 seeds plant-1) and 100% WHC (480 vs 417 seeds plant-1). Seed production for both species remained similar at 80 and 100% WHC; however, higher than 60% WHC. Seed production of A. fatua and A. ludoviciana was 235 and 282 seeds plant-1, respectively, at 60% WHC. The 60% WHC reduced seed production of A. fatua and A. ludoviciana by 51 and 32% respectively, compared to 100% WHC. The plant height, leaf weight, stem weight, and root weight per plant of A. fatua at 60% WHC reduced by 45, 27, 32, and 59%, respectively, as compared with 100% WHC. Similarly, the plant height, leaf weight, stem weight, and root weight per plant of A. ludoviciana at 60% WHC reduced by 45, 35, 47 and 76%, respectively, as compared with 100% WHC. Results indicate that A. ludoviciana can survive and produce seeds at 40% of WHC, indicating the adaptation of the species to dryland conditions. The results also suggest that A. ludoviciana is likely to be robust under water stress conditions, potentially reducing crop yield. The ability of A. fatua and A. ludoviciana to produce seeds under water-stressed conditions (60% WHC) necessitates integrated weed management strategies that suppress these weeds whilst taking into account the efficient utilization of stored moisture for winter crops.