A modelling framework incorporating relationships between agricultural production and groundwater hydrology was developed to estimate the benefits of improved irrigation efficiency in the Riverland of South Australia. Increased irrigation efficiency can generate external benefits to downstream users through reduced discharge of saline groundwater. In the Riverland these benefits are large in comparison to the direct value of the irrigation water. However, the non‐exclusive and site‐specific nature of these benefits makes it difficult to fully internalise them through market instruments such as salinity credits. Achieving optimal irrigation efficiency is likely to require institutional arrangements that promote collective investment and public expenditure.