Australia's New South Wales has logged its second-highest daily increase of the year in locally acquired Covid-19 cases amid fears of a wave of new infections after thousands of people joined an anti-lockdown protest.

"In relation to yesterday's protests, can I say how absolutely disgusted I was. It broke my heart," Gladys Berejiklian, the premier of the country's most populous state, told reporters.

There were 141 Covid-19 cases reported, down from 163 a day earlier. The outbreak, which began in June, is being driven by the highly contagious Delta variant, and has now infected 2,081 people in New South Wales.

"Let me be clear — there's not an alternative to the lockdown in New South Wales to get this under control. There is no other magic bullet that's going to do that," Prime Minister Scott Morrison told reporters at a televised media conference.

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