An epidemiologist is calling for an update to the country's six-month-old alert level system to include a requirement for masks to be worn on public transport under levels 3 and 4.
The Ministry of Health said on Friday a St Lukes mall worker may have contracted the virus by catching the same bus as another confirmed case, linked to the wider Auckland cluster that resulted in the city being put under alert level 3 restrictions last week.
Epidemiologist Michael Baker of the University of Otago says there's a high possibility the bus is the link between the two.
"Any indoor environment where people are quite close together, you can transmit this virus on respiratory droplets and aerosols that people generate just by talking, breathing and laughing," he told Newshub.
Auckland Transport is using AT Hop card data to contact trace others on the bus.
Prof Baker says masks should be mandatory on public transport.
"They haven't really been fully integrated into the alert level system yet because they're still voluntary, there's still a lot of confusion about where to wear masks."
A man was denied entry onto an Auckland bus on Wednesday because he wasn't wearing a mask, Stuff reports. Auckland Transport apologised, and said drivers had been told not to enforce such measures.
Masks work largely by preventing people's breath and droplets being spread around. The virus behind COVID-19, SARS-CoV-2, can be spread by people who don't even realise they're infected, hence health officials' urging of everyone to wear them when they're around people who aren't in their 'bubble' - even if they feel healthy.
Preventing the wearer from infection is a secondary benefit, and largely depends on the type of mask being worn - but even the most basic masks help stop people unwittingly spreading the virus, which has killed more than 800,000 people worldwide.
"I'd like to take this opportunity to remind everybody of the importance of wearing masks when particularly in places like public transport where it is difficult to maintain physical distancing," Director-General of Health Ashley Bloomfield said on Friday.
Prof Baker says mask wearing is currently erratic.
"We haven't really woven that into the alert level system in a very clear way. It's still a bit ambiguous for people."
New research has found the virus is likely able to travel through the air better in dry environments and those with air conditioning.
The Ministry of Health last week said it was seeking legal advice on whether making masks mandatory would be legal. Health Minister Chris Hipkins hasn't ruled it out.