(BSS/AFP) - Chemical testing has identified the mysterious black globules washing ashore on a popular Sydney beach as petroleum-based "tar balls", local officials have said.

Authorities were baffled earlier this week when thousands of black spheres -- sized between a golf ball and a tennis ball -- were found littering the shores of tourist haven Coogee Beach.

Tests showed the material was a "hydrocarbon-based pollutant" consistent with the "phenomena known as `tar balls`," the Randwick City Council said Wednesday evening.

Tar balls form when slicks of spilled oil are battered by wind and waves, mixing to form a sticky seawater emulsion that eventually breaks into smaller pieces or "balls".

It was still not known where the tar balls came from, council officials said.

The "mysterious, black, ball-shaped debris" began appearing on Coogee Beach on Tuesday afternoon, the local mayor said earlier this week.