Australia has an incredibly variable and unpredictable hydroclimate, and while irrigation is designed to reduce risk, significant uncertainty remains in both seasonal water availability (‘allocations’) and irrigation crop water requirements. This paper explores the nature and impacts of seasonal hydroclimatic uncertainty on irrigator decision making and temporary water markets in the Goulburn system in northern Victoria. Irrigation and water trading plans are modelled for the three seasons of the irrigation year (spring, summer and autumn) via discrete stochastic programming, and contrasted against a perfect information base case. In water-scarce environments, hydroclimatic uncertainty is found to be costly, in terms of both the efficiency of irrigation decisions and the allocation of water via the water market.