Thousands of black and grey debris balls, mostly the size of marbles, forced the closure of Bondi, Manly and other major Sydney beaches after washing ashore in late 2024 and early 2025.
The disgusting sight of workers in protective clothing shovelling effluent off pristine sands has prompted Friday's unveiling of a sweeping $3 billion investment in the city's water treatment infrastructure.
The NSW environment authority found the balls likely came from the Malabar deep ocean outfall, a wastewater system that services almost two million people in southern Sydney.
By upgrading treatment plants across the region, as much as 20 per cent of the wastewater usually pumped out at Malabar will be diverted for industrial use.
The upgrades could rectify Sydney's reputation as a city that barely treats its sewage, a water expert told AAP.
"My first reaction is 'bloody good'," Ian Wright of Western Sydney University's science school said.
"We have the three biggest ocean discharges of really only partly treated sewage in Australia."
Apart from stopping the flood of 'poo balls', Associate Professor Wright welcomes a concrete plan to provide the water needed for data centres - a pillar of a planned expansion of the state's tech sector.
"It's making all of us very nervous in the water industry, how much water is used in data centres for cooling," Assoc Prof Wright said.
"They have incredible thirst and it's going to get more and more, so I totally applaud that decision."
Australia's clean beach image is also at stake.
International tourists spent an estimated $53 billion in 2025, Tourism Research Australia data shows.
Domestic tourists' beach holidays were worth nearly $25 billion at last count in 2019.
"Beaches feature very prominently in the images that we use in promoting tourism (and) also the images in the minds of international visitors who are contemplating coming to Australia," Griffith University tourism expert Daniel Gschwind told AAP.
"The overall reputation we have as a clean destination ... we have a very keen interest in making sure we maintain that."
Staging the $3 billion upgrades over 10 years means Sydney Water users won't face bill shock and also paves the way for more housing to be built, Water Minister Rose Jackson said.
"People have got to be able to flush the toilets and turn the taps on, and this investment is going to unlock more housing right across southwest Sydney," she told reporters.
"If the Malabar system is indeed the source of those debris balls, and we can take a little bit of pressure off that system ... that's a real win, too."
Treatment sites in Glenfield and Liverpool will be the first to be upgraded in the coming months.
Work on the renewed infrastructure would begin immediately and education campaigns to stop people flushing away the harmful materials that form balls and block pipes would also ramp up, Sydney Water said.
"Don't put fats down the sink, don't pour your milk down there," chief executive Darren Cleary said.
"The three P's are the only things that should be going into the wastewater network."
19°C