Overflows into popular swimming spots can occur as a result of wastewater overflowing gully traps, manholes, engineered overflow points, or pump stations.
They occur when the wastewater system has been overloaded with water, typically after a heavy rainfall event, or due to a network error (such as a cross-connection).
It is recommended you do not swim for two days following a heavy rainfall event. This is to not only protect you from wastewater overflows, but also other contaminants from stormwater that wash off the roads.
If you want to know if it is safe to swim, check out the LAWA map here.