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.