Increasing demands for water from all sectors of the Australian economy means that irrigated agriculture (the single biggest user of water) has to develop more efficient ways of using the water that is made available. Farmers in many catchments are turning to on-farm water storages (OFWSs) as a means of capturing as much water as they can and to provide them with increased options for managing limited water supplies. The decision to invest in such storages is complex and multi-faceted, requiring assessment of a range of biophysical, economic, legislative and social factors. Computer-based simulation models can capture many of these factors and their interactions and, hence, can play a useful decision support role. In this paper, we describe a software package (Dam Ea$y) that couples biophysical and economic modelling tools, in a way that enables analysis of the likely costs and benefits of various scenarios involving investment in OFWSs. The paper includes a case study in which the yield, environmental and economic implications of investing in different sized OFWSs are considered for a representative sugarcane farm in Bundaberg, Queensland, currently operating with access to a small irrigation allocation from a nearby river and occasional 'out of allocation' water during periods of high river flow.