Sydney Water has advised it began discharging treated wastewater from the Bombo Treatment Plant from 5pm on Thursday.
Kiama Council advises swimmers and surfers to avoid using the northern end of Bombo Beach and Bombo Headland until further notice.
“An unknown contaminant from an external source has entered Sydney Water’s Bombo Wastewater Treatment Plant this afternoon,” a Sydney Water spokesperson said.
“All wastewater is continuing to be fully treated through the normal processes and we don't expect there to be large impact, however the local community may notice a discolouration in the water near the discharge point.
“Sydney Water has arranged for samples of the wastewater to be tested, is seeking to identify the contaminant and its source and will continue to update the local authorities and council on the progress of the matter. “
The current discharge from the outfall follows another episode in December 2017, when brown coloured water could be seen from the air.
At the time, Sydney Water explained the discolouration was likely caused by an equipment fault.
People wishing to report issues or obtain information can ring Sydney Water on 13 20 90.