Workers and residents in Sydney were ordered to stay home for a week on Friday, as authorities locked down several central areas of Australia's largest city to contain an outbreak of the highly contagious Delta variant of Covid-19.

Sixty-five Covid-19 cases have been reported so far in the flare-up linked to a limousine driver infected about two weeks ago when he transported an international flight crew from Sydney airport to a quarantine hotel.

But authorities have since identified scores of potential infection sites visited by thousands of people across central Sydney, including the city's main business district. 

Authorities have been alarmed by instances of people passing on the virus during fleeting encounters in shops and then quickly infecting close family contacts.

Premier Gladys Berejiklian of New South Wales state, which includes Sydney, called it the "scariest period" since the pandemic broke out more than a year ago.

On Friday, she ordered anyone who lived or worked in four central Sydney neighbourhoods to stay home for at least a week, only venturing out to purchase essential goods, obtain medical care, exercise or if they are unable to work from home.

The restrictions included central business district workers over fears that commuters were potentially spreading the virus into other parts of the city, Berejiklian said.

"We've done better than expected in terms of contact tracing and getting on top of all those links," she said.

"But what this does is make sure that we haven't missed any chains of community transmission."

An earlier ban on Sydneysiders leaving the city was also extended until next Friday, as traces of the virus were detected in sewage in the far-flung outback town of Bourke, about nine hours drive northwest of Sydney.

It was a dramatic development for a city that had returned to relative normality after months of recording very few local cases.

Australia Medical Association President Omar Khorshid chided New South Wales authorities for not taking tougher action, including locking down the entire Sydney metropolitan region, home to some five million people.

"The Delta virus is different; it is being transmitted far more easily," Khorshid told media in Canberra.

"Sydney has not faced this before."

Korshid warned that although the economic impact of a lockdown was hard, a wider outbreak could be "catastrophic" for the whole country.

It is the latest in a string of snap "circuit-breaker" lockdowns across major cities around Australia, with most cases linked to returning travellers held in hotel quarantine.

Australia has been among the world's most successful countries in containing Covid-19, with just over 30,000 cases and 910 deaths in a population of about 25 million.

Fiji says Covid-19 now widespread

Fiji health chiefs have admitted for the first time that Covid-19 transmission is widespread in the community after the Pacific nation recorded more than 300 new daily cases.

But authorities continue to reject calls for a nationwide lockdown, saying the 930,000-strong population's failure to obey health rules designed to contain the virus would render it ineffective.

Permanent Secretary for Health James Fong announced late Thursday that Fiji had recorded 308 new daily virus cases, taking the total number in a second wave of infections that began in April to almost 2,800.

Fong acknowledged calls for a lockdown but said Fiji's public compliance was low and authorities had no ability to enforce it, particularly in the densely populated shantytown settlements that were at most risk.

"The tragic reality is that Fijians living in communities most vulnerable to the virus –– including those in informal settlements –– are also those most vulnerable to the socioeconomic catastrophe that a 24-hour lockdown for 28 days would bring about," he said.

"Even under such a strict lockdown, we believe the virus would continue to circulate within many of these communities."

Fiji went a full year without recording any community cases up until April, when it was hit by a second wave of the quick-spreading Delta variant first identified in India.

As numbers continue to spike, doubling every nine days, the government's head of health protection, Aalisha Sahukhan, said "all the evidence is that there is widespread community transmission".

"With increasing cases, we expect increasing severe disease and deaths," she said. 

"While our hospitals right now are not overwhelmed with severe cases, from what we have seen in other countries with widespread community transmission during this pandemic, this is a very real possibility that we are preparing for."

Fiji's two largest hospitals have already been converted into dedicated Covid-19 facilities, with field hospitals set up to care for non-coronavirus patients.

The government's strategy is to use localised lockdowns to limit the virus while vaccinating the general population.

Only about one percent of Fijians have been fully vaccinated, with the Red Cross blaming misinformation spread online for the slow rollout.

President Joiji Konrote has urged his compatriots to get their jabs.

"Please think of our beloved nation and our people, take the vaccination," he said in an address posted online Friday.

"It is one of our main hopes of controlling the spread of the virus."