For days, thousands of black spheres -- ranging from the size of peas to tennis balls -- have appeared on the sand, puzzling locals and tourists alike.

Chemical testing has identified them as hydrocarbon-based tar balls, local officials said.

Tar balls can 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".

The source is still unknown.

"As a precaution, all Waverley Beaches will be closed until further investigation has been carried out," a local council that includes Bondi said.

The mayor of another beachside council, Dylan Parker, said three other beaches were also closed.

"Our beaches will remain closed and we are investigating," he said. "We are still waiting for the tides to come in."

While formally closed, access to the beaches has not been forcibly restricted, so some brave surfers and swimmers could still be seen in the water.

It is common for authorities to declare beaches closed during large swells.