Randwick City Council said preliminary tests indicated the material - which peppered Coogee Beach and was subsequently also found on Gordons Bay Beach - was a "hydrocarbon-based pollutant which is consistent with the makeup of tar balls".

Tar balls are formed when oil comes into contact with other debris and water - usually as a result of oil spills or seepage.

Both Coogee Beach and Gordons Bay Beach remain closed amid a clean-up to remove and dispose of the thousands of spheres the size of golf balls.

BEACH UPDATE: Gordons Bay Beach is closed after more mysterious, sphere-shaped debris were found washed up. Council is working closely with the @NSW_EPA to develop a clean-up methodology and safe disposal process for the debris. pic.twitter.com/9bOVCcFlXX

— Randwick Council (@RandwickCouncil) October 16, 2024

"Our community is rightfully very protective of our natural environment, and this has been a very concerning incident," Randwick mayor Dylan Parker said.

"Our priority as a local council is to keep people safe and protect as far as possible our coastal and marine environment."

Mr Parker said a specialist waste removal contractor was systematically removing the debris from the beaches, and the process would likely continue on Thursday.

Local authorities remain unsure about the scale or origin of the pollution incident, he said.

People were advised on Tuesday not to touch the debris at Coogee Beach, and more examples were found at Gordons Bay Beach on Wednesday.