Australia's most populous state, New South Wales (NSW), has reported its biggest rise in new locally acquired Covid-19 cases in nearly a week, prompting authorities to extend a mask mandate in Sydney for a week, reports Reuters.

Ten new locally acquired cases have reported in NSW, as officials fight to contain a latest cluster of the highly infectious Delta virus variant. Eight of the 10 are household contacts of previous cases in isolation.

"There is no doubt there is an increased level of concern, given the additional numbers of cases, but ... given how absolutely contagious the virus is, we expected household contacts already in isolation were likely to get the virus," NSW state Premier Gladys Berejiklian told reporters in Sydney.

Masks will be mandatory indoors in Sydney, Australia's largest city, for another week from Thursday morning although officials have stopped short of announcing further curbs as the cluster increases to 21 infections in six days.