"Mysterious, black, ball-shaped debris" began appearing on Coogee Beach on Tuesday afternoon the local mayor said, leaving flummoxed authorities scrambling to find out what they might be, and where they may have come from.

"Coogee Beach is closed until further notice", Mayor Dylan Parker said in a social media post.

Hundreds of golf-to-cricket-ball-sized spheres could be seen littering the length of sand, which is usually thronged with Sydneysiders and tourists.

Instead, a few seagulls wandered among the spheres, pecking and examining.

"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," Parker said.

The beach remained closed Wednesday morning despite an overnight cleanup effort.

Other nearby beaches were being monitored but remain open.

"Beachgoers are advised to avoid Coogee Beach until further notice and not touch the material, while the clean-up and investigations continue," Dylan Parker said.