Council workers collected the black balls into piles above the high-tide line on Tuesday evening and onto the back of a truck on Wednesday following advice from the NSW Environment Protection Authority. The Ports Authority and Transport for NSW are also part of the response.

An EPA spokesperson also took samples for testing on Wednesday, and a spokesperson said this was not yet complete.

While the scale or origin of the pollution incident is not yet known, lifeguards on jet-skis found what they believed to be a small dispersed oil slick off Coogee Beach on Wednesday morning. This masthead later saw a Port Authority vessel in the area.

Further north, Waverley Council beaches remained open and there were no reports of black balls. The NSW government’s Beachwatch website reported that the water was polluted and unsuitable for swimming at Bronte and Tamarama, but this was because of stormwater run-off.

Northern Beaches Council and Sutherland Shire Council both confirmed nothing had been washed up on their beaches.

Australian Marine Conservation Society oil and gas campaign manager Louise Morris said oil pollution in the ocean came from both offshore oil rigs and ships. Some petroleum products formed tar balls, while others were more likely to dissipate, but in either form it was a toxic pollutant that harmed marine life.

“We only see a small amount of what is going on because most of it will fall to the bottom of the ocean,” Morris said. “It’s out of sight, out of mind, but it’s being absorbed by marine life, some of which humans eat.”

Professor Ian Wright, a water scientist at Western Sydney University, said the source could be discharge from a ship, which burns diesel, or “an oilier, sludgier sort of fuel that’s more like asphalt”.

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He said similar pollution incidents could occur through the stormwater or sewerage systems, for example, if hot asphalt fell into a drain or if people put oil or grease down the sink and it formed “fatbergs” with solids in the wastewater.

The closure of Coogee on Wednesday interrupted daily swimmers and a PE class for Brigidine College Randwick.

Hans Visch, 76, is part of a group of swimmers who normally swim to Wedding Cake Island every morning. Visch saw news of the balls being washed up, but tried his luck on Wednesday morning, hoping it had reopened.

He was horrified by the pollution, given there were whales and other marine life in the area.

“It makes me really sad to see this happening,” he said.

Members of the public who find these balls in other locations should call the NSW Environment Line on 131 555.

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