The principles of seawater intrusion and purging of seawater out of deep tunnelled outfalls are well known. Purging using a relatively low sewage water discharge requires high hydraulic head loss, which conflicts with the requirement for minimum head loss in the diffuser to reduce energy consumption. The hydraulic modelling of purging processes for the Hong Kong outfall and diffuser system was performed in order to optimize the discharge and head loss requirements for purging. Special attention is paid to two different purging methods: two-layer purging and salt-assisted purging.