Beaches in Sydney, Australia have reopened after authorities removed thousands of mysterious black balls that tests revealed were formed by chemicals similar to those in beauty and cleaning products.

About 2 balls, some the size of golf balls, have been thrown from the sea across Sydney since Tuesday, forcing the closure of eight beaches, including the iconic Bondi Beach. Because of fears that they might be too toxic, a major clean-up operation was launched.

New South Wales marine executive director Mark Hutchings said the balls, which appeared in smaller numbers on Friday, were not harmful to health when left on the sand but should not be touched.

"We can now confirm that they consist of fatty acids, chemicals consistent with those found in cleaning products and beauty products and some fuel oil," he said.

The New South Wales Environment Agency said laboratory testing was ongoing to determine where the balls came from.

"It's still not clear where they're coming from and we need a few more days to figure that out," said executive director Stephen Beaman.

The beaches in Sydney, known for their golden sand and clear sea, attract tourists from all over the world.