Beaches closed until further notice included tourist hotspots Manly and Dee Why, the city's Northern Beaches Council said in a statement.
"So far, most samples identified are marble-sized with a few larger in size," it said.
The local government said it was alerted to the debris by the Environment Protection Authority, and was helping the state agency to collect samples for testing.
"The council is organising the safe removal of the matter and is inspecting other beaches," it said.
In October last year, spherical objects the size of golf balls appeared on the sands of Sydney's famed Bondi and Coogee beaches.
Initially thought to be tar balls, officials later said they could not be sure of the origin but they contained many substances including fatty acids, oils, human hair and bacteria.
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