Australia's two largest states have reported slight declines in new Covid-19 infections, prompting authorities to say it could be days before tough lockdown measures show progress in containing the spread of the Delta variant.

The country's most populous city, Sydney, and all of Victoria state — totalling nearly half the 25 million national population — are under stay-home orders after a flare-up of the highly infectious virus strain began last month.

New South Wales (NSW) state, of which Sydney is the capital, reported 105 new cases the previous day, down from 111 the day before, while Victoria logged 16 new cases, down from 19. NSW also recorded an additional death from the virus, taking the total to four since the start of the year and the national total to 914 since the pandemic began.

"Working together we will start to see those numbers nudge, and are throwing all of our resources and efforts to making that happen," said NSW Premier Gladys Berejiklian at a televised news conference.

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