Clovelly, Coogee and Maroubra beaches in Sydney will only open between 6am and 9am over the Anzac weekend after crowds ignored COVID-19 social distancing measures.
The decision to significantly restrict access to the patrolled beaches this weekend was made after too many people showed up on Friday, a Randwick City Council spokeswoman told AAP.
The council will reassess the situation on Monday.
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.
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 told AAP.
"We had put out a pretty stern warning to say even though it's 28C 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 warns lifeguards will 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," Mr Said said in a statement.
"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.
Nearby Waverley council beaches Bondi, Tamarama and Bronte will reopen for surfing and swimming next Tuesday, while Manly and Dee Why are the only northern beaches to remain closed indefinitely.
Australian Associated Press