Randwick's unpatrolled beaches - which include Gordons Bay, Malabar and Little Bay - will remain closed until further notice.
The council re-opened its beaches for exercise at the beginning of this week while warning against too many people gathering on the sand and in the water.
But they were closed on Friday afternoon after crowds descended "and people were unable to follow safe social distancing practices".
"By and large a lot of people are doing the right thing but what we found was in the middle of the day a lot of people were gathering on the beach, playing in the water and cooling down from the hot weather - these are not legitimate reasons to leave home," a council spokesman said.
"We had put out a pretty stern warning to say even though it's 28 degrees, it's not the day to come to the beach and have a fun leisurely day.
"You need to be surfing or swimming or jogging and doing some legitimate exercise."
The council had reopened the beaches partly because coastal walkways and local parks were being crammed with walkers and runners eager for space.
"Water is a legitimate place for people to exercise, it's important for their mental health," the spokesman said on Friday.
"We want to keep it open for exercise but we can't if people keep coming down for a fun day."
Lifeguards and rangers quickly dispersed the crowds and moved people on after the decision was made to shut the beaches.
Randwick Mayor Danny Said warned lifeguards would close the beaches again if safe social distancing is not followed.
"While we're expecting warm weather conditions this weekend, people should not be planning a day at the beach," he said.
"If residents head to the beach, it should be to go for a quick swim, surf or soft sand run, then home again."
The council's website makes clear people should limit groups to two people and leave the area once exercise has been completed.
"Sitting, sun-baking or gathering is not permitted," the notice states.
People in Australia must stay at least 1.5 metres away from others and gatherings are limited to two people unless you are with your family or household.
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