SYDNEY (REUTERS, XINHUA) - Australia's two largest states on Sunday (July 18) reported slight declines in new Covid-19 infections, prompting the 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 country's 25 million population - are under stay-home orders after a flare-up of the highly infectious virus strain began last month.

New South Wales state, of which Sydney is the capital, reported 105 new cases in the previous day, down from 111 the day before, while Victoria logged 16 new cases, down from 19.

New South Wales also recorded one more 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.

Premier Gladys Berejiklian of New South Wales refused to rule out the potential for further restrictions as case numbers in Sydney remained stubbornly high despite tough lockdown measures.

"I won't rule out tweaks in the next few days to make sure we haven't missed anything," she told reporters on Sunday. "Our aim is to quash the virus."

With 27 of the 105 cases listed as having been infectious in community, she said the authorities were intensely focused on seeing a reduction in that category, after numbers remained around similar levels for successive days.

"That is the number that worries us the most," she said of the cases that have not been isolating while infectious.

It has resulted in emerging hot spots across three of Sydney's local government areas, where the vast majority of the city's locally acquired cases have been reported. Communities in those locations were barred from leaving the area on Saturday as the government attempted to ring-fence the proliferation of the virus there.

Ms Berejiklian has also ramped up restrictions across Greater Sydney, including tougher rules applied to non-critical retail trading, while construction work was ordered to cease until July 30, when the lockdown order is due to end.

The state has recorded 1,242 locally acquired cases since the latest outbreak started a month ago, when an unvaccinated chauffeur infected with the Delta variant spread it while transporting airline crew.

"Working together, we will start to see those numbers nudge, and are throwing all of our resources and efforts to make that happen," she said.

"We want to make sure that we allow families and businesses in particular to bounce back as soon as we can and that is why we are throwing everything at it in the next two weeks."

A day earlier, Ms Berejiklian tightened restrictions on Sydney, a city of five million people, including a shutdown of all building and property maintenance works and bans on some 600,000 people in the worst-affected suburbs from leaving their immediate neighbourhood for work.

She said it appeared unlikely the number would be near zero by a July 30 target to lift restrictions.

She added that the number would likely begin falling in five days "because there is a lag in the data" and that she would not rule out changing restrictions further.

Neighbouring Victoria, with seven million people under a five-day lockdown scheduled to end on Tuesday, said all 16 new cases were linked to known chains of transmission.

Australia avoided the high infection and fatality numbers of many other countries in the initial stages of the pandemic due to swift border restrictions, stay-home orders and economic stimulus.

But a year and a half on, the federal government is under pressure due a sluggish vaccine roll-out, blamed by some experts on changing regulatory advice for the AstraZeneca vaccine and the limited supply of the Pfizer-BioNTech vaccine.

Just over 10 per cent of Australia's 25 million people are fully vaccinated, a fraction of the rates in the United States and Britain.

Meanwhile, the Royal Flying Doctor Service (RFDS) has been enlisted to inoculate thousands of Australians in remote areas against Covid-19.

Mr Frank Quinlan, the service's federation executive director, said it has administered some 5,121 coronavirus vaccines since being brought in by governments to accelerate the roll-out in remote communities.

Of those vaccines, it was estimated that about 45 per cent were for indigenous Australians.

"Early on in the pandemic, we identified a whole number of communities across Australia, where the RFDS is either one of or the primary source of healthcare," he said, according to Nine Entertainment newspapers on Sunday.

"So we're now provided vaccine clinics into those often remote and often small communities. We're expecting on the back of the plans we've worked up to be delivering some 50,000 vaccines to some 500 vaccine clinics between now and the end of the year."

The RFDS, a non-profit organisation, received A$38.8 million (S$38.8 million) in funding from the federal government to join the vaccine roll-out.

It has been tasked with delivering doses to remote medical centres, deploying support staff and supplying healthcare staff to administer jabs.

"The remoteness of some communities has been not only their protection, but also their risk, because we know that those communities are often protected by distance, but at the same time, they experience poorer health by distance," Mr Quinlan said. "We know if Covid-19 was to get into communities, the impact would be devastating."