In many cities and urban areas, policy makers and water providers are facing challenges of meeting human and environmental demands for water. Traditional centralized water management systems are failing to tackle these challenges due to their single-sectoral nature. Water sensitive urban design (WSUD) projects have been proposed as an alternative due to their ability to generate multiple benefits, such as water quality, water quantity, aesthetics, urban cooling and biodiversity benefits. However, investment in such systems has been rather low due to the lack of ability to assess and monetize the multifunctional benefits of WSUD projects. We describe the process of development, testing and application of INFFEWS (Investment Framework for Economics of Water Sensitive cities), an economic decision support system for investment in water sensitive designs that has been developed under the Cooperative Research Centre for Water Sensitive Cities Limited. INFFEWS is based on a benefit-cost analysis framework and is consistent with the principles and guidelines of the Australian national and state governments. The framework could be applied to business case development and decision making at multiple levels in public and private sector organisations in Australia and other countries facing similar situations.