Beaches in Sydney re-opened on Saturday for swimmers after authorities cleared thousands of black pollution balls washed ashore, which testing found to be formed from chemicals similar to those in cosmetics and cleaning products.

Around 2,000 of the mystery black balls, some golf ball-sized, had washed onto beaches across Sydney since Tuesday, forcing the closure of eight beaches, including the iconic Bondi Beach, and a massive clean-up operation, amid fears they were highly toxic.

"Based on advice from the Environment Protection Authority, we can now confirm the balls are made up of fatty acids, chemicals consistent with those found in cleaning and cosmetic products, mixed with some fuel oil," said New South Wales Maritime Executive Director Mark Hutchings in a statement.

He added the balls, which continued to appear on Friday in fewer numbers, are not harmful to health when left on the sand, but should not be touched.

The New South Wales Environment Protection Authority said laboratory testing was continuing, to try to determine where the balls came from.

"It is still somewhat of a mystery and may take a few more days to determine origin," said EPA Executive Director Stephen Beaman.

Sydney's ocean beaches, famed for golden sand and clean water, draw tourists from around the globe.

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