Wastewater System Faces Scrutiny After Coogee Tar Balls Mystery

Tar Balls in Coogee

The recurring appearance of mystery tar balls on Coogee Beach and other Sydney shorelines has intensified scrutiny of the city’s wastewater management, with experts attributing the issue to outdated sewage treatment systems.



Background: Recurring Tar Balls and Their Impact

The phenomenon first drew attention in October 2024, when black grime balls washed ashore on Coogee, Bondi, and other eastern beaches. Subsequent analyses revealed the balls contained human-generated waste, including fats, oils, hair, and traces of medications, suggesting a link to Sydney’s sewage outflows. Similar debris has since been discovered in the Northern Beaches, prompting temporary closures of nine beaches, including Manly, in January 2025.

The NSW Environment Protection Authority (EPA) and Sydney Water have launched investigations, but no definitive source has been identified. Some experts, however, argue the root cause lies in Sydney’s unusual reliance on primary sewage treatment.

Sydney’s Unusual Sewage System

Sydney’s wastewater plants at Bondi, Malabar, and North Head employ primary treatment, which only removes solids and surface grease. Unlike secondary and tertiary treatments, which are standard in many developed cities, primary treatment fails to filter dissolved contaminants and finer particles. The treated effluent is discharged into the Pacific Ocean via deepwater outfalls built in the 1990s, stretching 2 to 4 kilometres offshore.

Water treatment expert Stuart Khan has described Sydney’s system as “out of step” with other developed cities. He explained that fats, oils, and other materials can congeal in ocean water, potentially forming the tar balls washing ashore. Khan pointed out that heavy rains, which increase stormwater runoff, might exacerbate the issue by introducing untreated waste into waterways.

Coogee Tar Balls
Photo Credit: RandwickCC

Environmentalists’ Perspective and Historical Context

Environmental advocates, including Dr Richard Gosden and Professor Sharon Beder, argue that Sydney’s sewage system has long contributed to ocean pollution. In the 1980s, their group, Stop the Ocean Pollution (STOP), campaigned for secondary treatment at the city’s wastewater plants but lost out to a cost-saving approach that prioritised extending outfalls over upgrading treatment processes. Gosden claims the issue has been downplayed by authorities for decades, stating, “It’s been conducted in diluted sewage all this time.”

Beder, who has studied the history of Sydney’s sewerage engineering, maintains that the government must improve treatment standards and address industrial waste entering the sewers. She says the combination of faecal matter, oils, and other materials found in the balls strongly implicates the city’s wastewater practices.

Broader Investigations

The EPA has confirmed that the tar balls contain materials associated with sewage but has not ruled out other sources, such as illegal dumping or ship waste. Chemical analyses of the black grime balls found hydrocarbons similar to shipping fuel, adding complexity to the investigation. Experts like Dr Sharon Hook from the CSIRO believe the balls likely form when fats and oils mix with seawater, and their movement depends on ocean currents.

Sydney Water has engaged external experts to trace the source of the pollution and is exploring long-term infrastructure upgrades. A $30 billion investment plan has been announced to modernise the city’s wastewater systems over the next decade, though many believe more immediate action is needed.

Impact on Communities and Environment

The ongoing issue has disrupted beach activities across Sydney, a city known for its iconic coastline and beach culture. Local councils, supported by the EPA, continue cleanup efforts while warning beachgoers to avoid touching the debris. Residents and environmentalists are calling for greater transparency and faster action to address the pollution.



The tar balls incidents have reignited debates about Sydney’s wastewater management and its long-term impact on marine environments. Experts and advocates agree that upgrading sewage treatment processes to include secondary and tertiary methods could help prevent similar events and protect the city’s coastal ecosystems.

Published 21-January-2025

Mobile Ad