The balls washed ashore on the eastern end of Silver Beach at Kurnell on Tuesday.
The NSW Environmental Protection Agency (EPA) was alerted by a member of the public who spotted the objects.
Sutherland Shire Council set up warning signs along the impacted section of the beach, advising the public about the suspected contamination.
Council workers yesterday started cleaning up the affected stretch of sand.
A search of nearby beaches by officials failed to find any similar debris.
The balls varied in size, shape and colour, with some rounded and golf-ball size while larger ones were more irregular in shape.
They range in colour, from whitish or pale, through to green, grey and black.
The EPA said the amount of mysterious balls and debris found this week was much smaller than what washed up on beaches in Sydney in October.
Tests conducted by Randwick City Council established the debris was a phenomenon known as “tar balls”.
A tar ball is a small blob of oil that has been weathered and shaped after floating in the ocean for a period of time.
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Examinations by the EPA confirmed earlier results that indicated their origin was likely a source that releases mixed waste.
But scientists could not determine where the balls originated from because no source samples were available for comparison.
The EPA is awaiting results of testing on debris balls which washed up in Kiama, on the NSW South Coast, last month.
