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 and 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 Sydney's unique, outdated sewage management system has been linked to "debris balls" which have been washing up on beaches with increasing frequency, including at Malabar last week. Experts say dirty waters can attract more sharks, as the nutrients attract plankton, which in turn attract the fish sharks feed on. So, is there any link between Sydney's sewage system and the "unprecedented" number of shark attacks in a short space of time? Here's what experts say.
What do we know about the shark attacks? The shark attacks happened on the 18th, 19th, and 20th of January at Vaucluse, Dee Why, and Manly in Sydney, and on the mid-north coast. NSW Minister for Primary Industries and Regional Development, Tara Moriarty, described this month's shark attacks as "unprecedented." Dr Amy Smoothey analyzed images of the bites from the recent attacks and concluded that bull sharks were likely involved. Bull sharks prefer water temperatures above 19C and tend to arrive in Sydney in October, with their numbers peaking in January and February. Experts have attributed 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 the intense rainfall seen in Sydney last weekend, where some areas received over 50mm in a few hours, as a key factor behind the attacks. They say the rainfall flushed out brackish water from estuaries into the ocean, which suits bull sharks because they can hunt in low visibility. The murky waters also attract "bait" fish, who feed on microorganisms like plankton, which thrive on nutrients and bacteria from river, storm water, and sewage overflows.
So, could Sydney's unique sewage system have played a part? Professor Ian Wright, a water scientist at Western Sydney University who worked for Sydney Water, believes so. After the four attacks in quick 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 is unique in Australia as it only uses primary treatment, which removes solids before releasing effluent directly into the ocean. Other cities use secondary treatment, which involves biological processes to break down organic material. Sydney's system is also very 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 increase the risk of drinking water and environmental contamination. Wright suggests that storm water could have flushed Sydney's diffuse pollution and "sewage from overflows from a sewage system that needs upgrading" into the ocean, potentially driving shark activity. "A lot of our urban runoff, especially in heavily developed areas, becomes dilute sewage. Bacterial levels rise," he explained.
Shark Ethology Australia consultant and academic Victoria Camilieri-Asch believes 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 then attract small fish, leading to larger fish and eventually more predators. When asked about Sydney's sewage system, Camilieri-Asch acknowledged the possibility, noting that bull sharks are known to stay in areas with wastewater in other countries. What about the deepwater ocean outfalls? Another unique feature of Sydney's sewage system is its deepwater ocean outfalls, built in the late 1980s and early 1990s. Three treatment plants at Malabar, Bondi, and North Head pump 80% of the city's effluent through pipes extending 2 to 4km out to sea. A Sydney Water report from August 2025 linked heavy rain and fat buildup at the Malabar plant to the formation of "poo balls" being ejected from the deep ocean outfall, which washed up on Sydney beaches last summer and again after the weekend storms. However, Robert Harcourt, an emeritus professor of marine ecology, believes it's a "long shot" to link the deepwater ocean outfalls to the recent shark attacks near beaches, as the "nutrification" process that attracts microorganisms and sharks has been pushed offshore since the outfalls were built. Wright also disagrees, but Dr Colette Kerry, a physical oceanographer, suggests that current coastal conditions could mean the ocean outfalls are having an impact. Climate change has made the eastern Australian current warmer and closer to the coast, trapping nutrient-rich, brackish water from rivers and estuaries, and potentially the effluent discharged from the outfalls.
What don't we know? Guardian Australia sought clarification from Sydney Water about the potential impact of its wastewater management practices on shark activity. The utility referred questions to the NSW Minister for Water, Rose Jackson, who issued a statement after the attacks. She cautioned against oversimplifying the issue, emphasizing that heavy rainfall is common, and shark attacks in Sydney and NSW waters are very rare. Jackson assured that Sydney Water takes water quality seriously and is investing $34 billion in network upgrades and expansions over the next decade, including at Malabar, Bondi, and North Head systems. Sydney Water's CEO, Darren Cleary, defended the use of deep ocean outfalls, stating that they have met environmental standards for nearly three decades. He also noted that Sydney beaches generally have good water quality and that Sydney Water conducts regular testing around the outfalls.
This week's attacks have sparked questions about wastewater systems and their management, according to Camilieri-Asch. Wright believes the warnings for swimming water quality and shark safety are now effectively the same. He advises, "If it's poor water, if it looks dodgy, if it smells a bit off, don't go in. You could pick up a waterborne disease, but you could also become prey for a shark."
