Flowrate changes in separate wastewater sewerage during wet weather are often attributed to inappropriate connections of runoff water. But these inputs are not the only cause of such behaviour and infiltration through leaking defects or cracks can supply large flows into collection pipes. The characterization of these flows is not an easy task since they involve complex processes, depending not only on rainfall events but also on hydrological conditions and seasons. Using 16 months of hourly measurements of rainfall and flowrates, we developed a six parameter conceptual model to simulate rainfall induced infiltration into a small sewer system. Sensitivity analysis applied to the proposed model showed good achievements under various calibration conditions but displayed high parameter interactions which may be a serious drawback for some specific model uses. Model application to another sewer network yielded good agreement between observed and simulated flows. The suitability of this model must be further checked on other sites.