Warnings have been issued to Australians over the highly infectious Delta strain with one epidemiologist describing positive cases are "walking into a cloud of infectious particles".
Expectations within health experts indicate New South Wales will see more cases as more exposure sites were announced overnight, including in Bondi and on two "flights of concern".
Those flights were into and out of Wellington and the Ministry of Health has issued an alert here.
Authorities are now concerned over how easily the variant of the virus has been spreading, with NSW Premier Gladys Berejiklian announcing a week-long extension of mask measures yesterday.
Mary-Louise McLaws, professor of epidemiology at the University of New South Wales and an adviser to the World Health Organisation, described the spike in numbers as "enormous" and said the Delta strain is mutating like never seen before.
While McLaws praised the decision on masks, she revealed some terrifying realities of the new variant which has the world watching over the "fleeting" manner in which the virus is spreading.
Talking to the ABC's Gemma Veness, McLaws said the variant means people become more infectious earlier than that of its virus predecessor, which backs research from Chinese doctors who say that up to 12 per cent of patients are becoming severely or critically ill within three to four days of the onset of symptoms.
In the past, the proportion had been only 2 or 3 per cent of those infected.
"This variant of concern means that you can start to become more infectious to people earlier on, so we know that you're very infectious with any variant or wild strain at about day 5 when you become symptomatic but certainly from day 3 onwards you can of course still infect people," McLaws said.
"With this particular Delta it looks as if you are very much more infectious."
She suggested Sydney's Bondi cluster and fear over exposure in its Westfield Bondi Junction complex could be because shoppers are walking into a "cloud of infectious particles".
Thousands who visited Bondi Westfield over the space of a week will need to get tested as NSW health authorities raise their level of concern of the venue.
Berejiklian said the shopping centre had generated "a number of cases" and authorities wanted to make sure they had "captured everybody".
Anyone who visited the centre, including the carparks, at any time between June 12 and 18 has been asked to come forward.
McLaws explained: "Normally you need at least a few minutes to inhale enough because we've seen mostly you acquire covid from large droplets."
"However, the more sadly this pandemic continues the more we're learning that in the different phases of the disease you are more likely to exhale smaller particles than larger particles.
"So perhaps what we're seeing is people walking in shopping malls indoors — where there's not a lot of air flow change for a highly infectious disease — and they're exhaling small particles that don't fall to the ground fast and so you're walking into a cloud of infectious particles."
McLaws theorised that it's not the "fleeting moment of crossing paths" but more "a fleeting moment of crossing a plume of infectious aerosolised particles".
Eric Liang Feigl-Ding, an American public health scientist, used Sydney's example of the Delta variant, telling his more than half a million followers "this kind of 'fleeting contact' infection is all the more reason for masks and improved ventilation and air disinfection".
"I think it's safe to say Delta Variant is now the greatest threat of Covid-19 pandemic in 2021. It is by far the fastest variant known to date. The data compiled from 64 countries by WHO don't lie. B16172 is a clear and present danger to the world."
There are currently 21 infections in NSW's Covid-19 outbreak.
Berejiklian said that "at this stage, we feel the response we are having is proportionate to the risk" – but NSW Health Minister Brad Hazzard warned residents to "take it very, very seriously", especially because it's the more infectious Delta variant.
"I'm very pleased to hear that mask use is now mandatory not just mandatory in transport because that's what will save us from walking into these plumes of infectious particles," she said.
She claimed children were even more susceptible to catching the strain, "than we've ever seen before".
"Delta is showing us that children can catch this, before I was always very positive that children didn't catch Covid except if they were a household contact.
"But this is different. This Delta has now worked out how to infect children."