Billions will be spent over the next decade to upgrade a city's sewerage network after debris balls washed up on iconic beaches.
Thousands of the black and grey balls, mostly the size of marbles, forced the closure of Bondi, Manly and other major Sydney beaches in late 2024 and early 2025.
The NSW environment authority found the balls likely came from the Malabar deep ocean outfall, a wastewater system that services almost two million people.
The network will be upgraded to reduce the volume of wastewater that needs to be treated and discharged, under a program expected to cost $3 billion over the next 10 years.
The staged upgrades will ensure Sydney Water users don't face sudden bill shock and pave the way for more housing to be built, Water Minister Rose Jackson said.
"Sydney is a rapidly growing city and no one wants to see debris balls washing up on our beautiful beaches again - but the truth is our wastewater system needs an upgrade to keep pace with the population," she said.
"For too long, a lack of investment in essential infrastructure in western Sydney has been a handbrake on our housing goals.
"We can't undo that overnight, but we're getting on with the job of clearing this backlog."
The debris balls may have formed due to an increased load of fats, oils and grease over time combined with oceanographic factors and weather conditions playing a role, the investigation revealed.
Test results revealed hundreds of materials from household fats to human hair made up the balls.
No single origin point was identified, but the balls likely appeared over summer as a result of 10 or more separate discharge events.
Sydney Water chief executive Darren Cleary said the plan would reduce the increasing strain on the "ageing coastal plant".
"We understand the seriousness of recent debris ball incidents and the need to ensure our network is equipped for Sydney's growing population," he said.
Treatment sites in Glenfield and Liverpool will be the first to be upgraded in coming months.
In the short term, Sydney Water is increasing its cleaning and inspection of ocean outfall screens, tightening controls for high-risk customers and expanding its fats, oils and grease education campaign.
