Australian authorities extended a lockdown in Sydney on Wednesday by at least 14 days, after three weeks of initial restrictions failed to stamp out an outbreak of COVID-19 in the country's largest city.

New South Wales state Premier Gladys Berejiklian said restrictions would need to remain in place until at least July 30 after reporting that there were 97 new locally transmitted cases, representing a slight increase in daily infections from a day earlier.

"It always hurts to say this, but we need to extend the lockdown at least a further two weeks," Berejiklian said in Sydney on Wednesday.