Coogee, Maroubra and Clovelly beaches in the city's east remained closed on Friday as workers in orange hazmat suits continued to clean up the washed-up material.
It was the second day of the operation following the initial discovery of the pollution on Tuesday.
But several beaches further north, including Bondi, were reopened after being shut off to the public on Thursday.
NSW Environment Minister Penny Sharpe said she was very worried that Sydney's beautiful beaches had been left polluted by the material, which was still of unknown origin.
"We're investigating to try and find the source of the spill and who is responsible, and I would hope that we can do that," she told reporters.
"I would also hope that, if we can, we would throw the book at those involved."
Bondi's reopening on Friday coincided with the launch of the annual Sculpture by the Sea exhibition, which spans the coastline from there to Tamarama to the south.
Waverley Council said a site inspection had turned up no evidence of the debris on its beaches, but staff would monitor the situation closely to protect the community.
The public have been urged not to touch the balls, the largest of which have been washing up at Coogee and its neighbouring beaches.
Tests by the NSW Environment Protection Authority previously confirmed the potentially hazardous material was a "hydrocarbon-based pollutant, which is consistent with the make-up 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.
EPA executive director Stephen Beaman said there was likely a low threat to human safety and fewer balls were washing up on each tide.
A chemical analysis was being carried out to identify the main material involved and that would direct further investigations, he said.
The limited number of affected beaches suggested the source could have been relatively close to shore, rather than an oil tanker out at sea, experts have said.
NSW Maritime sent boats out to investigate but they found no telltale sheen on the water suggesting an oil spill.
