The imbalance of a coastal water supply and demand system is temporally affected by both natural and anthropogenic processes that are interactions between interdependent components with many feedbacks. Water availability is influenced over time by interactions among sea level rise, river flows and salt-water intrusion. Water demand is affected by population growth, agricultural and industrial production. The interactions of these factors will lead to increases or decreases in the imbalance of the coastal water supply and demand over time. Understanding temporal interactions of these drivers and their effects on the imbalance of the coastal freshwater system over time is necessary to improve coastal freshwater management. This study proposes a new approach that applies a system dynamics modelling tool to investigate the imbalance between water supply and demand over time in a highly populated coastal area, Hai Phong, Vietnam. More specifically, this paper investigates the effects of sea level rise, river flows and salinity intrusion on a coastal freshwater availability, together with the effects of population growth, agricultural and industrial production on water demand. The interrelationships among interdependent variables causing the imbalance of the coastal water supply and demand system are depicted by a “Drifting goals” system archetype with two balancing loops. Both two goal seeking loops (B1 and B2) are capable of relieving the imbalance of the system. However, the goal seeking loop B1 generally takes more time and efforts to achieve a goal than the goal seeking loop B2. This conception plays an important role for stakeholders in identifying management interventions for the coastal freshwater system. A preliminary imbalance model of the coastal water supply and demand system was developed. The key data requirements and equations for calibration and simulation of the model were also initially identified. A number of methods will be used to reduce uncertainties inherent in, and validate the performance of the models. The uncertainties associated with climatic and socio-economic unpredictability will be reduced by developing a range of scenarios of sea level rise, river flow, and domestic use, agricultural and industrial production. The validity of the models will be calibrated by using expert and stakeholder judgements, historical data, previous hydrodynamic studies, and field measurements of river flows and salinity in the study area. This study has a high potential for novelty as it incorporates both climatic and non-climatic drivers into one framework to assess the imbalance between coastal water supply and demand in a developing country. More specifically, it will provide an understanding of the present and future imbalance of a water supply and demand system in highly populated coastal areas with a high rate of urbanization and population growth, together with sea level rise and precipitation decrease. This study will directly contribute to the development of collective and decisive adaptation actions that aim to secure efficient freshwater resources for the socioeconomic development in the port city of Hai Phong, Vietnam.