The spill was first reported on Wednesday afternoon, and the source of the leak was later tracked down to a valve on a 24-inch city sewage main near the Aliso and Woods Canyon Wilderness Park.
On Friday, six miles of beaches were re-opened.
The remaining areas are still closed until testing shows the water is back to acceptable standards of cleanliness.
One Orange County health officer said exposure to untreated sewage “can be harmful and result in very serious illness with potentially severe effects”.
Depending on the intensity of the rain and the volume of runoff, the water could remain at dangerously contaminated levels for up to three days.
The advisory will be in effect until at least Monday morning, but could be extended depending on rainfall.