A lockdown in the Australian city of Sydney has been extended until the end of September to slow the spread of a Covid outbreak.

Authorities also imposed a curfew on two million residents in the city's worst-hit suburbs.

Residents of Sydney have been under stay-at-home orders since late June.

However, infections have more than doubled in the past week with 642 new cases recorded on Friday, and 681 on Thursday.

"I apologise to the vast majority of people in those communities who are doing the right thing but for our health and safety moving forward we need to make these difficult decisions," New South Wales (NSW) Premier Gladys Berejiklian said.

She added that the curfew - which will run from 2100 to 0500 - was aimed at "reducing the movement of young people", because police said there had been instances of rule-breaking late at night.

But critics say there's little medical evidence to show that curfews are an effective virus control - a view also expressed by the Ms Berejiklian last month.

Local lawmakers have also criticised authorities for applying harsher restrictions in the city's poorer west and south-west suburbs.

The toughest rules target Sydneysiders living in 12 council areas of concern - which are also the city's most ethnically diverse communities.

Along with the curfew, people in those suburbs will be limited to just one hour of exercise daily.

The measures were designed to prevent more people from "losing loved ones", the premier said.

Critics say the state government could have acted earlier to tackle the outbreak of the highly contagious Delta variant.

Australia on Thursday marked its highest daily infections since the pandemic began, recording 754 cases.

Currently over half of Australia's 25 million people are living in lockdown, amid outbreaks in Melbourne and Canberra.

Sydney remains the biggest concern - 65 deaths have now been recorded in NSW as a result of the latest outbreak.

The rest of NSW is also in lockdown, but Ms Berejiklian said restrictions outside Sydney would be eased on 28 August.

So far, Australia has recorded 971 deaths from Covid, fewer than many nations.

But Delta's entry into Australia in early June has challenged a nation initially widely praised for its pandemic response.

Frustration has built over strict border policies and frequent lockdowns.

Residents in Melbourne on Thursday marked their 200th day of lockdown since the pandemic began.

Critics have also accused the federal government of complacency over its slow vaccine rollout.

Prime Minister Scott Morrison's government is under significant pressure, and has scrambled to secure more doses.

Vaccinations have picked up in recent weeks. More than a quarter of Australians are now fully vaccinated.