The warning from Brian Turner, the manager of the Dunedin City Council's Tahuna wastewater treatment plant upgrade, came after an Otago Daily Times photographer spotted the brown scum at St Clair on Saturday, and again yesterday.
Mr Turner said members of the public often raised concerns about the unsightly substance accumulating on beaches. Some feared it was raw sewage.
However, the material was a naturally-occurring mixture of algae and other organic debris from seaweed, other plant-life and crustaceans, which could be "exacerbated" by nutrient run-off from land, he said.
A "scum line" could appear off the coast where two currents met, particularly after strong southerly winds and large swells, and come ashore in concentrations along the shoreline, he said.
Toxic algal blooms were also known to occur in warmer waters further north, but the material accumulating on Dunedin's beaches was not a threat to swimmers, who "tend to move through it", he said.
However, dog owners were advised to keep their pets clear.
"It's not particularly nice ...it is organic matter and a lot of it's dead organic matter, so it's not really a good idea to let your dogs near it, really."
Information on the council's website also showed scum samples had been tested for faecal contamination in the past, usually after public complaints, but only "very low" concentrations of bacteria had been detected.
