Swimmers and surfers were urged out of the water on Tuesday afternoon after the white-grey, ball-shaped debris was found along Sydney's northern beaches.
The discovery comes three months after Bondi and other beaches in Sydney's east were closed when 2000 black tar balls, which were found to be a mix of human hair and household fats washed up over several days.
The pollution watchdog's analysis of the new balls was ongoing.
"It could be different but the MO is very similar," water scientist Ian Wright told AAP on Tuesday.
"It may not be the case (that the source is sewerage) but it needs to be treated with appropriate caution ... it is reasonable that the beaches are closed," Associate Professor Wright said.
Northern Beaches Council said it was alerted to the issue by state environment officials.
"So far, most samples identified are marble-sized with a few larger in size," a Northern Beaches Council spokesperson said.
Nine beaches from Manly to North Narrabeen were closed.
Some of the debris discovered on Tuesday was collected for testing while the council pledged to safely remove the rest.
Nine beaches from Manly to North Narrabeen were closed after the discovery of the debris. (Northern Beaches Council/AAP PHOTOS)
Other beaches, including Collaroy, Fishermans and Narrabeen, were also due to be inspected.
While the city had experienced several days of heavy rain in the past week, Sydney Water said it had no issues with the normal operations of the local wastewater treatment plants, processing about 1.1 million people's waste.
"We comply with our licences as set by the NSW Environment Protection Authority (EPA) and only discharge compliant wastewater during normal operations," a spokeswoman said.
"Sydney Water is continuing to work with the EPA to investigate the cause of the grease balls."
But repeated occasions where wastewater is causing pollution and with no clear source must be addressed by the pollution watchdog and Sydney's water authority, Greens environment spokeswoman Sue Higginson said.
"We need to have a serious conversation about the accountability of utilities like Sydney Water, and we must do better when it comes to taking responsibility for subjects like where our crap goes," she said.
The NSW EPA was contacted for comment.
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