SYDNEY (BLOOMBERG) - As Perth and Brisbane join Sydney and Darwin to become the latest Australian regional capitals forced into a business-crippling lockdown, Prime Minister Scott Morrison's balancing act of keeping Australians safe and the economy open is becoming increasingly precarious.

His government's decision 15 months ago to close international borders was pivotal to Australia's success in curbing community transmission, allowing life to largely return to normal and the economy to bounce back.

But a tardy vaccine roll-out that has seen just 23 per cent of the population receive its first jab has made the nation particularly vulnerable to the highly contagious Delta strain that leaked into the community earlier this month, likely from a driver infected while transporting international flight crew.

And with Australia now in the grip of the Southern Hemisphere winter, health experts say the nation potentially faces months of restrictions just as economies with more advanced vaccine programmes, such as Britain and the United States, prepare to open up.

With the Delta strain and the way that it is spreading and infecting many people, it is "becoming very critical" to ramp up the vaccine roll-out, Dr Taghrid Istivan, an associate professor of microbiology at Melbourne's RMIT University, said in an interview. Until the majority of people are jabbed, "this will continue to happen".

In a late-night press conference in Perth on Monday (June 28), Western Australia state Premier Mark McGowan announced a snap four-day lockdown after a woman returned to the city from Sydney while infectious.

The outbreak in Sydney now numbers nearly 150 cases, with 19 more infections announced by New South Wales state on Tuesday. Contact tracers are battling to keep up with a growing list of exposure sites, including some domestic Virgin Australia flights after a cabin crew member tested positive.

Residents of Greater Sydney have been ordered to stay home except for exercise, essential shopping and medical treatment until July 9, while Darwin, the capital of the Northern Territory, on Monday saw its initial 48-hour lockdown extended until Friday.

Queensland state has imposed additional restrictions, while Perth and Canberra have made mask-wearing mandatory in public for the first time since the pandemic began.

'Scariest period'

"Since the pandemic has started, this is perhaps the scariest period that New South Wales is going through," Ms Gladys Berejiklian, the Premier of Australia's most populous state, told reporters last week.

Mr Morrison, who is self-isolating after travelling to Europe this month to meet leaders attending the Group of Seven summit, has lauded the government's record in tackling the virus.

"The world is amazed at how Australia has been able to both save lives and save livelihoods," he told viewers of a popular morning-television programme last Thursday. "We are keeping our economy growing and we're keeping people safe."

But criticism is mounting.

Tourism operators are warning that the localised lockdowns are playing havoc with their industry, while the Australian Retailers Association says the Sydney stay-home order is expected to cause lost trade totalling about A$1 billion (S$1 billion) a week.

Ahead of elections due to be held by May, the main opposition Labor party is on the attack, criticising the government for being too slow in securing vaccines and not reaching out to enough drugmakers.

Labor also blames Mr Morrison for failing to improve a quarantine system in which residents returning from overseas spend 14 days in mandatory isolation in city-centre hotels. Labor says there have been 24 instances in which the virus has leaked into the community via infected security guards or cleaners.

Travellers have also caught the virus while in quarantine, possibly due to poor ventilation systems, and only tested positive after being released.

"They had two jobs this year: vaccination and quarantine," Labor leader Anthony Albanese told Parliament last month. "They have botched both."

Should more cities enter lockdown and restrictions linger, that could have real political implications. A Newspoll published in the Australian newspaper on Monday showed Mr Morrison's conservative coalition slipping 2 percentage points to trail Labor, 49 per cent to 51 per cent.

"The perception is becoming increasingly one of a government managing crisis, not the nation," said political analyst Andrew Hughes at Canberra's Australian National University.

The slow vaccine roll-out - along with criticism of its policies on gender equality and climate change - means "the government is going from a likely winning position at the next election to where there are now real doubts if its record on economic management will be enough to keep it from losing office", he said.

Infectious diseases specialist Peter Collignon at ANU's Medical School said the hotel quarantine system and shuttered international border had been very successful in curbing the spread of the virus.

Still, for at least the next three months until the end of winter, "we are going to have to live with restrictions of some degree in order to get control of this", he said.