This paper addresses a method for determining priority areas for ensuring and maintaining the suffi-cient reliability and efficiency of existing water supply and distribution systems, based on previously obtained mathematical models of water sampling (cross-section method) and flow distribution. The main task of the organisations, operating water supply and distribution systems, involves a high-quality water supply to consumers. Regardless of the wear of water supply and distribution systems, as well as changes in their structural state, the water having suitable parameters must be supplied to the con-sumer in the required quantity. Given the variety of regimes, modelling the operation of water supply and distribution systems indicates a wide range of their possible behaviour, for example, poor quality of supply to consumers during trouble-free operation or no impact of shutdowns of some sections on the quality of supply. It is necessary to analyse the systems and model their operation regimes using a mathematical tool. Here, probabilistic nodal reliability indices of water supply to consumers were used in mathematical models. They include Kj and Pj, decomposed into components characterising the wear of elements (sections) in the system and the reliability of supply to consumers in each of its structural states. The need to adjust the system parameters to ensure the required level of reliability of its opera-tion was assessed, with various solutions being outlined. The technical and economical comparison of possible measures allows an optimum option to be determined, prioritising the replacement of outdated elements and the construction of additional elements (sections) of the system. All stages of the sug-gested method, which can be widely used by organisations, maintaining water supply and distribution systems, are shown in the example of a system.