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.

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.

Nine beaches along Sydney's Northern Beaches have closed for precautionary reasons after numerous tiny balls were found washed up on the shoreline.

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.

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

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

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.

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.

Nine beaches in Sydney’s north have been closed after mysterious balls of debris washed ashore. The marble-sized balls of debris washed up on Tuesday, the Northern Beaches Council said, prompting the closure of nine popular beaches.

A number of northern beaches have been closed after ball-shaped debris was found in the sand, following similar balls being discovered on eastern suburbs beaches last year.