Two popular Sydney beaches closed after mysterious black balls appear

Two popular eastern Sydney beaches were closed until further notice after the appearance of "mysterious black" balls, local authorities said.

Coogee beach, particularly popular with backpackers, was cleared on Tuesday after lifeguards "discovered mysterious, black, ball-shaped debris washed along the length of the beach," the Randwick city council said in a statement.

"At this stage it is unknown what the material is, however, they may be 'tar balls' which are formed when oil comes in to contact with debris and water, typically the result of oil spills or seepage," the statement continued.

On Wednesday, Gordons Bay beach, just north of Coogee, was also closed until further notice as debris "after more mysterious, sphere-shaped debris were found washed up," the council said.

The local authority said that environmental officers have collected samples which were being tested. It added that arrangements were being made to safely remove the material, as it advised the public to avoid the beach until further notice.