Two popular beaches in Sydney, Australia were closed off to the public until further notice after hundreds of "mysterious black balls" appeared, local authorities said.

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

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

Beachgoers have been told to stay clear while the 'black balls' are removed. (Deutsche Welle)

Australia investigates beach mystery

Local authorities advised the public to avoid the beaches as environmental officers carried out tests and the material was being removed.

Initial tests showed the material was a hydrocarbon-based pollutant consistent with the makeup of tar balls.

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

Upon the discovery of the objects, ranging in size from golf to cricket balls, the New South Wales Environment Protection Authority (NSW EPA) was immediately alerted.

In a social media post, Randwick city mayor Dylan Parker said the council was working closely with the NSW EPA to develop a clean-up methodology and safe disposal process for the debris.