A Sydney wastewater treatment plant has been identified as the likely source of thousands of mystery balls that closed beaches last summer, according to reports.

Several beaches in Sydney’s eastern suburbs — including Coogee, Manly and Botany Bay — were closed after “poo balls” washed up between October 2024 and February 2025.

Information now points to them being created by a combination of so-called “fatbergs” lining wastewater pipes and heavy rainfall flushing the gunk through the pipes.

An oceanographic modelling report by WQ Data, commissioned by Sydney Water, suggested heavy rainfall coursing through pipes was dislodging fats, oils and greases from the sides of the pipes.

Wave action then rolled the material into balls and carried them to various coastal areas on currents, according to the report obtained by The Guardian.

The NSW Environmental Protection Agency said on Friday that Sydney Water’s Malabar wastewater treatment system had been identified as the likely source.

The EPA told The Guardian it had recommended Sydney Water carry out further investigations and actions to pinpoint potential sources of the debris within the Malabar system.

“There are global issues about fatbergs and sewers, and it’s effectively the same thing,” EPA advisory panel chair Professor Stuart Khan said.

“This particular sewer [at Malabar] has had a long time for fats, oils and grease to build up.

“There’s nearly two million customers in that system, and over the past 10 years, not only has there been growth in population using that system, but also increasing numbers of food outlets that may or may not have appropriate trade waste licences in place or the right infrastructure in terms of grease traps and things to be preventing runoff of fats, oils and grease.”

Testing last year was conducted by the NSW Environment Department and University of NSW scientists.

Fatty acids from cooking oils, soap and skincare products, as well as petroleum hydrocarbons, were found alongside hair, food waste and other naturally decomposing substances.

At the time the EPA said they likely came from a source of “mixed waste” involving hundreds or even thousands of different materials.

Sydney Water confirmed it had found no issues with the operation or with maintenance on wastewater facilities near the beaches at the time.

Analysis of weather patterns in the lead-up also revealed no conclusive information.

The washed-up waste was originally thought to be “tar balls”, which form when oil comes into contact with debris in water.

That would suggest a spill or seepage from a ship as the source of the pollution. Testing instead showed similarities to “fatbergs” that can form in and clog sewerage with household waste.

Sand, calcium salt and bacteria commonly associated with wastewater were also identified during analysis of the balls.

-with AAP