One of the challensges for sewerage development in South East Asia is unexpected low-strength and low-flow influent to domestic wastewater treatment plants, which can cause mismatching between the plant design and influent characteristics. One of the reasons of this unexpected influent characteristics is poor sewer networks in the region. For effective development of sewerage in the region, this study aimed to estimate the change in influent sewage characteristics according to development stages of sewer networks. Taking into account of various conditions of sewer networks, we developed a model to estimate sewage flow, sewage pollution loading, and sewage concentration, employing unit pollution loadings of toilet wastewater and greywater. The model was tested in a residential drainage area of Hue city, Vietnam, and validated by comparing the measured sewage polllution loading and the model-estimated sewage pollution loading at present conditions. The validated model was appied for the same residential area at four development stages of sewer networks. Results indicated that sewer leakage reduction mainly would contribute the increase of sewage flow; an integrated measure of sewer leakage reduction, appropriate greywater discharge to sewers, and septic tank removal would contribute the increase of sewage concentration.