Could Sydney's creaking sewage system be linked to the spate of shark attacks? After four shark attacks in New South Wales in less than 48 hours, authorities on Tuesday urged beachgoers to stay away from the water, suggesting they instead visit local pools. This isn't the first time Sydney has faced such warnings; in the past, they've been issued for beaches polluted with faecal matter after heavy rains. Sydney's unique, outdated sewage management system has been linked to "debris balls" washing up on beaches, including at Malabar, with increasing frequency. Experts say dirty waters can attract sharks, as nutrients attract plankton, which in turn attract the fish that sharks feed on. So, is there a connection between Sydney's sewage system and the unprecedented number of shark attacks? Here's what experts say.
What do we know about the shark attacks? The attacks occurred on January 18, 19, and 20 at Vaucluse, Dee Why, Manly, and on the mid-north coast. NSW's Minister for Primary Industries and Regional Development, Tara Moriarty, described this month's attacks as "unprecedented." Dr. Amy Smoothey analyzed bite images and concluded that bull sharks were likely involved. Bull sharks prefer water temperatures above 19C and typically arrive in Sydney in October, with peak numbers in January and February. Experts attribute the attacks to a "perfect storm" of factors, including more people in the water during summer, rising ocean temperatures due to global heating, and murky waters after weekend storms. Many experts point to intense rainfall as a key factor, with some areas receiving over 50mm in a few hours. This rainfall flushed brackish water from estuaries into the ocean, which suits bull sharks as they can hunt in low visibility. The murky waters also attract "bait" fish, which feed on microorganisms like plankton, thriving on nutrients and bacteria from river, storm water, and sewage overflows.
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So, could Sydney's unique sewage system have played a part? Professor Ian Wright, a water scientist at Western Sydney University with experience at Sydney Water, believes so. After four attacks in close succession, Wright was perplexed, asking, "Why did this happen?" He identified three critical factors: extraordinary rain, warm water, and very poor water quality. Sydney's sewage management system, unique in Australia, uses only "primary treatment," removing solids before releasing effluent into the ocean. Other cities use secondary treatment, which involves biological processes to break down organic material or remove nitrogen and nutrients. Sydney's system is old, with aging pipes, plants, and sewers, some dating back to the 19th century, struggling to keep up with population growth. During extreme rain events, storm and wastewater overflows can contaminate drinking water and the environment, potentially driving shark activity.
Wright suggests that storm water may have flushed Sydney's diffuse pollution and "sewage from overflows from a sewage system that needs upgrading" into the ocean, attracting sharks. Shark Ethology Australia consultant and academic Victoria Camilieri-Asch agrees that the link between storm, agricultural, and wastewater runoff following heavy rain and increased shark activity is "indirect." She explains that these nutrients can provide food for microorganisms, which attract small fish, leading to larger fish and more predators.
What about the deepwater ocean outfalls? Sydney's sewage system features deepwater ocean outfalls, built in the late 1980s and early 1990s. Three treatment plants pump 80% of the city's effluent through pipes extending 2 to 4km offshore. A Sydney Water report from August 2025 linked heavy rain and fat buildup at the Malabar plant to "poo balls" being ejected from the outfall, washing up on beaches last summer and after recent storms. However, experts like Harcourt and Wright disagree, viewing the outfalls as an unlikely direct cause of the shark attacks.
Dr. Colette Kerry, a physical oceanographer at the University of NSW, suggests that current coastal conditions may be impacting the outfalls. Climate change has made the eastern Australian current warmer and closer to the coast, trapping nutrient-rich, brackish water from rivers and estuaries, potentially including effluent from the outfalls.
What don't we know? Guardian Australia sought clarification from Sydney Water about the potential impact of wastewater management practices on shark activity. The utility referred questions to the NSW Minister for Water, Rose Jackson, who cautioned against oversimplifying the issue, emphasizing that heavy rainfall is common, and shark attacks in Sydney and NSW waters are rare. Sydney Water's CEO, Darren Cleary, defended the use of deep ocean outfalls, stating they've met environmental standards for nearly three decades. He assured that Sydney beaches generally have good water quality, and the utility is investing in a $34 billion network upgrade and expansion over the next decade.
However, Camilieri-Asch and Wright highlight the need for further research into the relationship between wastewater systems and shark activity. Wright warns that poor water quality, poor visibility, or an off smell should prompt beachgoers to stay out of the water, as it may indicate waterborne diseases or shark activity.
