A third Australian state announced lockdown rules to combat the Covid-19 Delta variant spread, with South Australia entering week-long restrictions, joining an extended lockdown in Victoria and a five-week shutdown in Sydney.

Australia's most populous state New South Wales (NSW), of which Sydney is the capital, is battling the worst Covid-19 outbreak of this year, with total cases exceeding 1,400 since the first case was reported more than a month ago in a limousine driver who transported overseas airline crew.

NSW authorities reported a slight slowdown in new cases on Tuesday as infections fell to 78 from 98 a day earlier as Sydney and some surrounding areas endure a fourth week of lockdown.

At least 21 of the new cases were infectious while still in the community, a number that authorities said must be close to zero in order for lockdown restrictions to be lifted.