Underground sewage pipe systems deteriorate over time resulting in cracks and joint defects. Sewage thus leaks out and contaminates the surrounding groundwater and the surface water in stormwater drains. Many studies have investigated the problem of sewage leakage but no published studies, to the best knowledge of the authors, have examined the hydrologic interactions between leaky sewage pipes, groundwater and stormwater drains. This study numerically models such interactions using generic conditions in Singapore. It first develops accurate representations of weep holes and leaky sewage pipes, and further shows the long-term and short-term system responses to rainfall events. Some of the implications include: (1) quality of water seeping into the drains tends to be low in dry years; (2) complete contaminant attenuation after pipe rehabilitation takes several years; (3) responses to rainfall events at weep holes are immediate but the effects on sewage leakage might only show up a few days later. The simulation results allow us to better understand the local-scale migration of sewage leakage from a sewage pipe to nearby stormwater drains. With calibrations and verifications with local field data, the modeling framework would be applicable and beneficial to the sewage leakage monitoring and sewage pipe rehabilitation worldwide.