Apparent losses consist of water volume drawn from the network, consumed by users but not paid for. Those due to water meter under-registration were evaluated by means of a mathematical model able to analyse the complexity of intermittent supply systems with private tanks. This supply scheme is very common in the Mediterranean area where unexpected water shortage often happens and intermittent water supply is a common practice. In order to analyse such complex systems, a demand model, reproducing the effect of private tanks, and an apparent losses module were developed and coupled with an hydraulic network model based on the Global Gradient Algorithm (GGA). In distribution networks pressure reduction valves (PRVs) are often used by water utilities to control the pressure and reduce background losses. These practices could influence the performance of water meters. For this reason, a PRV model was implemented and integrated with the demand and the hydraulic network models to better estimate the effect of pressure management on real and apparent losses. The comprehensive model was applied to a real case study. The proposed modelling approach was used to identify regions of the network with high apparent losses. Furthermore, the model may be used to predict the results of a water meter replacement plan and of the installation of devices that could affect apparent losses.