Swimmers and surfers were urged out of the water on Tuesday afternoon after white-grey, ball-shaped debris was found along Sydney's northern beaches.
The 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.
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 source of that pollution was never found.
Nine beaches from Manly to North Narrabeen were closed after the discovery of the debris. (Northern Beaches Council/AAP PHOTOS)
Some of the debris discovered on Tuesday has been collected for testing while the council will safely remove the rest.
Other beaches, including Collaroy, Fishermans and Narrabeen, are also due to be inspected.
The midsummer surprise comes during the school holidays and the mercury hovering in the high 20s.
The NSW Environment Protection Agency was contacted for comment.
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