Australia's most populous state recorded no new Covid-19 infections for a third straight day but extended raised social distancing and mask-wearing rules by a week as the authorities hunted for the source of a small outbreak.

“As the 'missing link' case hasn't been identified we're keen to prevent a super-spreading event,” said New South Wales state premier Gladys Berejiklian in a tweet.

“All safeguards/restrictions will be in place for an extra week, except for shoppers in retail who will no longer be required to wear a mask.”