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.

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

The discovery of the marble-sized balls comes after a similar incident in October closed Sydney's Bondi Beach, among other tourist hotspots.

Nine beaches in Sydney were closed on Tuesday as authorities investigated marble-sized balls of white and gray debris that had washed on shore.