This report provides design details and costs for the proposed long-term improvements to the water supply system at Hihifo on the island of Tongatapu, Kingdom of Tonga. This is part of the demonstration project under the Pacific Adaptation to Climate Change (PACC) programme. The proposed project aims to provide an opportunity for the Hihifo community to use local water resources sustainably and, hence, enhance the resilience of these water resources to the impacts of current climate variability and projected climate change. The present public water (or ‘town water’) supply system at Hihifo is based on groundwater pumped directly into the distribution system and has shown a high level of unreliability and poor water quality for a number of years.