Bondi Beach was shut after thousands of mysterious tar balls washed up on Australian shores in recent days.
The popular tourist attraction was among seven beaches that were totally, or partially, shut in Sydney on the east coast.
Tar balls are formed when oil comes into contact with debris and water, often as a result of oil spills or seepage - although the authorities admitted they do not know where they were coming from.
Local councils began their clean-up efforts on Wednesday night.
Tests confirmed the material is a "hydrocarbon-based pollutant", one council said, after they were first discovered on 15 October.
Bondi Beach, Bronte Beach, Tamarama Beach were closed by Waverley Council, while Clovelly Beach, Gordon's Bay, Coogee Beach and Maroubra Beach, were shut by neighbouring Randwick Council.
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"The safety of the community is our first priority," Randwick mayor Dylan Parker said.
"That's why [the] council has taken the precaution to close these beaches while we undertake the safe collection and disposal of this material.
"We don't yet know what has happened to produce the debris washing up on our beaches, but we will continue to work with relevant authorities to ensure the safety of the public and clean up our beaches."
New South Wales state's Environment Protection Authority said it was conducting tests and advised people against swimming near or touching any of the balls.
Sydney is home to more than 100 beaches in its harbour and along its oceanfront. They are famous across the world, attracting millions of tourists and locals each year.
(c) Sky News 2024: Mystery tar balls wash up on Sydney beaches forcing seven to close
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