The new Water Management Act (2000) requires water to be specifically allocated for environmental purposes to try to improve river health. Water sharing plans are being developed which establish extractive and environmental shares to river flows. A key consideration in the development of these plans is the economic trade-offs associated with different allocation options. In unregulated catchments, allocation rules typically involve significant restrictions to extractive access at times of low river flows, which also coincides with periods of environmental stress. The highly variable nature of stream flows in unregulated catchments creates challenges not only to the development of plausible sharing rules but also to the assessment of their effects. In this paper proposed water sharing rules in the Mooki sub-catchments in Northern NSW is examined. A combination of representative farm (linear programming) and hydrology simulation modelling is used to explore these effects.