Nine beaches in Sydney, including well-known Manly beach, were closed to bathers on Tuesday after small white and grey balls of debris washed up on the shores at the height of the summer holiday season.

Sydney’s popular Manly Beach and other beaches around the Australian city were closed on Tuesday as authorities investigated marble-sized balls of white and gray debris that had washed on shore, in the latest such mysterious incident in recent months.

All but one of the nine major north Sydney beaches which were closed this week after mysterious ball shaped debris washed up on the sands have now reopened.

Sydney’s pumping of primary-treated effluent into the ocean makes us out of step with the rest of the world, a water engineering expert said.

Beaches in Sydney, Australia, were closed to bathers after small white and grey balls of debris washed up on the shores at the height of the summer holiday season.

Workers have cleaned up and reopened seven of nine northern Sydney beaches after mysterious balls washed ashore.

Months after mysterious black balls forced the closure of some of Sydney's most famous beaches, small marble-like debris has begun washing up on the city's shores again. The balls - this time grey or white in colour - have prompted councils to shut nine beaches, including popular Manly and Dee Why, while authorities investigate.

But repeated occasions where wastewater is causing pollution and with no clear source must be addressed by the pollution watchdog and Sydney's water authority, Greens environment spokeswoman Sue Higginson said.

The Northern Beaches Council said that efforts were underway to safely remove the debris. Meanwhile, Local authorities collaborated with the state’s environmental agency to investigate the matter.