Millions of Sydney residents will spend another month in lockdown, authorities have announced, citing a still fast growing coronavirus outbreak and stubbornly low vaccination rates.

Australia's biggest city had been due to exit five weeks of lockdown on July 30, but the restrictions will now remain in place until August 28 as case numbers continued to climb, reports AFP.

“I appreciate personally what we're asking people do for the next four weeks but it is because we want to keep our community safe and want to make sure we can bounce back as quickly as possible,” New South Wales state premier Gladys Berejiklian said.

Officials announced 177 new cases linked to the Sydney outbreak, which began mid-June when a driver for an international flight crew contracted the virus.