Sustainable development of floodplains is closely linked to sustainable flood mitigation measures. Various sustainability assessment (SA) methods to evaluate the influence of policies, plans or projects aimed towards sustainable development have been emerging in recent years. However, most of them are at the national or regional level. Very few research studies have been carried out for sustainability assessment of flood mitigation projects. This study proposes a new innovative decision support framework for sustainability assessment (SA) of flood mitigation projects throughout the project life cycle, focusing on two main aspects: sustainable flood mitigation by the project, and enabling of sustainable development of the floodplain. This study has employed a review of the life cycle of flood mitigation projects, a review of sustainability assessment methodologies, consultations with experts and case studies involving two flood mitigation projects in Queensland, Australia. Conforming to the project life cycle, the decision support framework for sustainability assessment of flood mitigation projects is developed incorporating five stages: 1) contextualizing the project with regard to floodplain sustainability, 2) SA during planning and implementation for integrating sustainability issues in the project, 3) SA during a flood event to assess the sustainability performance of the project 4) SA at periodic intervals, and 5) SA at the stage of modification or changing to a new project. The framework has adopted a multi-criteria analysis (MCA) approach using sustainability criteria and indicators to determine the sustainability index for the project. The process of selecting indicators, defining the weightages and scores for indicators, and determining a sustainability index for various stages of the project has been described in this thesis. The application of the SA framework to the first two stages of the two case study flood levee projects demonstrates how the best suitable alternative levee option can be chosen in the planning stage by determining a sustainability index (SI) of the possible alternatives using a set of sustainability indicators. The study also shows achievement towards sustainability of the finally implemented project can be compared with the originally planned project using the SA framework. The application of the SA framework suggests the potential for better decision making for individual flood mitigation projects, taking into account the sustainable outcomes of the project as well as linking these to sustainable regional development. The outcome of this study will enhance decision making for sustainability of flood mitigation projects. Adapting the framework to projects in other development sectors is also envisaged.