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.

Eight beaches including Bondi were closed for several days in October and a massive clean-up ordered after thousands of black deposits started appearing on the coast.

Testing by authorities determined those balls were most likely the result of a sewage spill.