Australia's Covid-19 response team holds urgent meeting amid outbreak

* Treasurer says Australia facing 'critical time'
* NSW warns cases could rise 'considerably'
* Queensland, South Australia announce restrictions

Australia's Covid-19 response committee is due to hold an emergency meeting on Monday as outbreaks of the highly contagious Delta variant across the country prompted a lockdown in Sydney and renewed restrictions elsewhere.

More than 20 million Australians, or around 80% of the population, are now under some form of lockdown or Covid-related restrictions as officials grapple with Covid-19 flare-ups in almost every state or territory.

‘I think we are entering a new phase of this pandemic, with the more contagious Delta strain,’ federal Treasurer Josh Frydenberg told the Australian Broadcasting Corp on Monday, adding Australia was facing a ‘critical time’ in its fight against Covid-19.

The national security committee, chaired by Prime Minister Scott Morrison, would be briefed by the country's chief medical officer later on Monday, Frydenberg said.

Sydney, Australia's most populous city and the capital of New South Wales (NSW) state, began a two-week lockdown over the weekend.

Eighteen new local cases were reported in NSW on Monday, compared with 30 a day earlier, taking the total infections in the latest outbreak to 130 since the first case was detected nearly two weeks ago in a driver for overseas airline crew.

‘We have to be prepared for the numbers to bounce around and we have to be prepared for the numbers to go up considerably,’ NSW Premier Gladys Berejiklian told reporters in Sydney.

‘With this strain, we are seeing almost 100% transmission within households.’

An initial two-day lockdown in the northern city of Darwin, scheduled to end on Tuesday, was extended to Friday after the Delta variant of the virus was found in a fly-in, fly-out mine worker.

Queensland and South Australia reintroduced mandatory masks and restricted home gatherings, following a similar move by Western Australian officials for state capital Perth. Restrictions remain in place in Victoria state capital Melbourne and national capital Canberra.

Queensland reported two new locally acquired cases while Western Australia and Northern Territory detected one each.

A health alert was issued over the weekend for hundreds of passengers after an infected Virgin Australia cabin crew member worked on five flights covering Brisbane, Melbourne and the Gold Coast.

Australia has so far fared much better than many other developed countries in tackling the spread of the coronavirus, with just over 30,500 cases and 910 deaths.

Lockdowns, tough social distancing rules and swift contact tracing have helped suppress prior outbreaks but the fast-moving Delta variant has alarmed authorities.

NSW police fined 44 people for breaching stay-at-home orders, including a pair of naked sunbathers who became lost in a national park after being startled by a deer.