For the past year, Australia has been coasting along almost blissfully detached from the global pandemic. It had achieved a "Covid normal" where people could visit restaurants and nightclubs and join crowds at festivals and theatres.
The country's strong Covid defences - border closures and mandatory quarantine - worked 99.99% of the time.
When cases did leak, officials acted quickly, aggressively locking down cities and hunting down every infected contact.
Sydney - Australia's biggest and richest city - managed to avoid regular snap lockdowns helped by a "gold standard" contact-tracing system.
But in the past fortnight, the Delta variant has managed to breach the city's defences. In one week, positive cases have ballooned to more than 100.
By Friday 25 June, officials conceded the need to put Sydney into lockdown. By the following Monday, the crisis had became a national one - with outbreaks in four states and territories.
Sydney, Darwin, Perth and Brisbane- all capital cities - are now in lockdown.
More than 20 million Australians, around 80% of the population, are living under restrictions - the highest number since a national lockdown at the start of the pandemic.
In an emergency meeting on Monday, federal and state governments attempted to plug the holes by expanding vaccine access.
But many Australians are asking why they're back living under restrictions, seven months after the world started mass vaccinations
Where Delta saw the weak spots
Epidemiologists say the Delta variant has proven to be the most infectious and transmissible of all the strains so far.
Where it saw cracks in Australia's defence system, it succeeded in exploiting them.
The nation's border and quarantine system had been increasingly challenged since the first variants emerged in late 2020.
Officials documented cases where travellers were catching the virus in quarantine, despite staying in separate rooms.
Experts raised concerns about air recirculation and the lack of fresh air in city hotels.
Around 370,000 people have gone through the system. But there have been 10 breaches leading to outbreaks.
Two troubling Australia right now came from people who served hotel quarantine. One is a mine worker in the Northern Territory who caught the virus in Queensland quarantine. The other is a Queensland woman whose infection surfaced just as she completed her stay.
The other weak spot is workers at the border.
Australia has been notoriously strict on gatekeeping who's allowed into the country - with weekly limits on the number of returning citizens and outright bans from some virus hotspots.
When they step off their plane, returnees are greeted by an intimidating coterie of soldiers, police officers and nurses - masked-up and gloved to escort arrivals straight to quarantine.
But the same rigour isn't applied to other workers - like drivers transporting arrivals.
Patient Zero in the Sydney outbreak was a limo driver in his 60s who caught it from a passenger. He was neither vaccinated, wearing a mask nor being tested regularly - and he didn't need to under the rules at the time.
Despite these flaws, experts note that Delta is a "formidable foe" due to its high infection rates.
In New South Wales, of which Sydney is the state capital, officials are reporting near 100% household transmission compared to 25% for earlier strains. People there have caught the virus just from passing one another in a shop.
"Delta is just extremely, highly contagious. And even with the vaccinated workforce there's still potential to transmit," says Prof Nancy Baxter, Head of the School of Population and Global Health at the University of Melbourne.
She points out that prior to the outbreak, officials had seen Delta cases where "they can't even identify how the transmission occurred".
"So I think even when the systems are perfect, it's challenging. But the systems aren't perfect, which just kind of makes us almost sitting ducks."
Unvaccinated and exposed
Delta's danger has also shone a light on failures in Australia's vaccine programme. Just under 5% of the adult population is fully vaccinated, with 29% receiving a first dose.
Australia is last among OECD countries, when it comes to the rollout of vaccines. Critics say the government is responsible.
