Large sandstone blocks have been placed across one entrance as council crews close eight kilometres of beaches and car parks from Kyeemagh to Taren Point Bridge.
Most Cook Park car parks have gates with padlocks, so closing them has been a relatively simple exercise.
The sandstone blocks were lifted into position on Wednesday at the car park entrance closest to the Cooks River at Kyeemagh.
Mayor Joe Awada, who announced the decision by Bayside Council on Monday, said the move was already proving effective, with a drop in the number of people coming to the beachside.
"I have had a couple of people call me to question what was going on, but there hasn't been any pushback in terms of people not accepting it," he said.
Cr Awada said the move was in line with the appeals about social distancing being made by federal and state leaders and senior health officials.
Walking and cycling paths along the edge of the bay remain open, with reduced numbers of users expected as result of the car park closures.