From 15 minutes to 5 seconds to pass on COVID-19

It took just seconds for a person to be infected with COVID-19 as a stranger walked past them while shopping at Bondi Junction Westfield in Sydney.

And the ABC reports it hasn't happened just once.

Australian health officials are warning it no longer takes up to 15 minutes to pass on the deadly virus.

It can be a "fleeting moment" of just five to 10 seconds.

Viruses like coronavirus mutate over time as they spread throughout the population.

These are known as variants.

Two of those variants that have spread in the community in the past month have been the Kappa variant and the Delta variant.

The Kappa variant is highly infectious and triggered Victoria's latest lockdown.

The state's COVID-19 commander Jeroen Weimar said the Kappa variant had spread in cases of "fleeting contact".

While COVID previously infected people in the same house or workplace, Kappa has "stranger-to-stranger transmission".

The Delta variant was also detected in Victoria after the start of lockdown and is the variant in New South Wales' outbreak.

Experts say the Delta variant appears to be more transmissible across all age groups, including children.

It is spread so easily that a person in Sydney infected a stranger simply by walking past them.

The Delta variant appears to be more infectious than other variants, more resistant to health controls and causes more varied and severe symptoms.

NSW Chief Health Officer Kerry Chant described the Bondi Junction incident as a "scarily fleeting" encounter.

Dr Chant says they are clearly facing each other but it is literally someone moving across from each other for a moment, close, but momentary.

Dr Chant suspects two other people were infected with COVID in the same fashion.

She says they haven't been able to look at the exact same crossover point [in those two cases], but they know they were 20 metres, signing in at different venues at the same time or in that area, so they suspect they did cross over.

Griffith University virologist Lara Herrero says in the case that was captured on CCTV, the virus would have had lingered in the air long enough for the unfortunate person to take a breath in and be infected.

The speed and ease with which the Delta variant spreads has prompted states and territories to act quickly to shut travellers from affected areas out.

Queensland's Chief Health Officer Jeannette Young has warned they are seeing "very fleeting contact" in some cases.

She says at the start of this pandemic, she spoke about 15 minutes of close contact being a concern.

Dr Young says now, it looks like it's five to 10 seconds.

She says that's a concern and the risk is so much higher now than it was only a year ago.