Australia is grappling with a fresh COVID-19 crisis as nearly every state in the country has recorded outbreaks of the highly infectious Delta variant of the virus in recent days, prompting authorities to again impose lockdowns or other restrictions.

Over the weekend, more than 5 million people in Sydney, the capital of the state of New South Wales, entered a two-week lockdown, and New Zealand suspended its "travel bubble" arrangement, putting the brakes on trans-Tasman travel.

Within Australia, a growing sense of alarm has seen some states announcing closure of their borders for people from other regions.

With officials saying that Australia is facing a "critical time" in its battle against the pandemic, questions are mounting as to how the more contagious strain of the virus has spread so quickly on the continent, and why mass-vaccination rollout has been slow in Australia when compared to similar campaigns in the United States and the United Kingdom.

As of now, only around 4 percent of Australians have been fully vaccinated, or received both doses, against the COVID virus, while in the UK the corresponding figure is 47 percent and in the US, the number is at 46 percent.

Chris Moy, vice-president of the Australian Medical Association, told the Nine television network on June 28 that "we are basically having a cross-country seeding event, that's as simple as that."

He said the states and territories must go hard on restrictions early to minimize the spread after two separate coronavirus outbreaks sparked cases in New South Wales, the Northern Territory, Western Australia, and Queensland.

Mark Butler, opposition health spokesman from Australia's shadow Cabinet, said millions of Australians were paying the price for Prime Minister Scott Morrison's mistakes with the vaccine rollout and hotel quarantine.

"More than a year after the pandemic began, two capital cities (Sydney and Darwin) are now in lockdown and restrictions are being reimposed" in other places, Butler told a media briefing on June 28.

"This is why we need more vaccines. This is why we need national quarantine."

According to Australia's health minister, Greg Hunt, more than 7.32 million vaccines have been administered across the country to date with 28.5 percent of eligible Australians now covered by first doses.

Experts point out that the figure is much lower if you take into account only those who have received both doses of the vaccine.

New South Wales Premier Gladys Berejiklian told a media briefing on June 28 that the state needs more vaccines to prevent future outbreaks.

"No matter how good your system is, it is not until the vast majority of the population is vaccinated that we will be able to protect our citizens. It is the bottom line," she said.

A limousine driver at the center of the Sydney outbreak drove aircrew to and from Sydney airport to quarantine hotels in Sydney. He was not wearing a mask or tested for COVID-19. As of June 28, he has not been charged.

Ian Henderson, director of the Institute for Molecular Bioscience at the University of Queensland, said unless Australia stops being complacent "we will continue to have virus escape into the general population that result in lockdown periods."

"Those who understand the history of infectious diseases understand this," said Henderson who was the founder director of the UK's Institute of Microbiology and Infection.

"Quarantine areas need to be distant from large urban centers, where workers in the quarantine system do not interact with people from large urban settings on a frequent basis."

It is essential that the population is vaccinated as quickly as possible to offer the greatest chance of eradicating the virus and drive a return to normalcy, Henderson said.

Abrar Chughtai, director of the Master of Infectious Diseases Intelligence program in the School of Public Health and Community Medicine at the University of New South Wales, Sydney, said: "Throughout the pandemic, there's been a discussion around slow vaccination in Australia and breaches in hotel quarantine."

"Due to slow vaccination, most of the Australian population is still vulnerable. We need to increase the vaccination rate rapidly," he said, noting that "Australia is really behind in vaccine rollout, even compared to many low-income countries".

Michelle Grattan, Professorial Fellow at the University of Canberra, told the news outlet The Conversation that New South Wales "is on a knife edge — the coming days will show whether this will be a nasty but brief pause on activity ... or something much more serious.

"Many breaches of hotel quarantine have occurred — although given the number of people passing through, and the fact hotels are not ideal for the purpose, perhaps the surprise is there haven't been more."