Health experts from the World Health Organisation say there is not yet enough evidence to confirm that the Delta COVID-19 variant can spread in only five seconds rather than 15 minutes.
The Delta variant - first detected in India - is behind clusters in Sydney, Melbourne and Queensland.
Australian health officials have flagged that the virus can spread in the amount of time it takes to breathe in.
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Dr Mike Ryan, director of WHO's Health Emergencies Program, said it was scientifically possible that the virus mutated to become "fitter" as it overtook previous mutations.
"It can shift the infectious dose, in other words the virus may be more efficient at infecting cells and you need less virus to cause an infection," Dr Ryan told a virtual news conference from the WHO headquarters in Geneva today.
NSW Chief Health Officer Kerry Chant has said a person infected with the Delta variant at Sydney's Bondi Junction Westfield caught the disease in a "scarily fleeting" encounter, according to CCTV footage.
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But Dr Ryan cautioned that more research was needed into the Delta variant.
"How much virus do you need to be contaminated by or inhale before you reach a dose that causes you to have an infection? That's not even known for the previous strains and it's not fully understood for these newer strains - Delta strain included," Dr Ryan said.
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His comments follow WHO director-general Tedros Adhanom Ghebreyesus' warning yesterday that the Delta variant is "the most transmissible of the variants identified so far," and warned it is now spreading in at least 85 countries.
Tedros, an Ethiopian who goes by his first name, said the lack of vaccines in poor countries was exacerbating the Delta variant's transmission. He described a recent meeting he attended of an advisory group established to allocate vaccines.
"They were disappointed because there is no vaccine to allocate," he said, criticising rich countries for declining to immediately share shots with the developing world. "If there is no vaccine, what do you share?"
The Delta variant is behind Sydney's ballooning cluster.
NSW has recorded 29 new cases of COVID-19 with Premier Gladys Berejiklian "putting everyone on notice" that a wider Sydney lockdown could be imposed this weekend.
- With AP
