Epidemiologist Professor Michael Baker's "big message", if Delta is in the Auckland community, is to wear a mask because it's transmitted mainly through air particles.
The Otago University Professor's warning comes after the Ministry of Health announced on Tuesday afternoon it was investigating a community case of COVID-19 in Auckland, though details are scarce and it's not yet known if it's the Delta variant.
Prof Baker told Newshub the reason the Delta variant - which originated in India - is spreading so rapidly, is because it "generates a lot more viral particles in your upper airwaves when you get infected, even if you don't have symptoms".
"It means that when you're speaking or coughing or singing or laughing - all those things you might do when you're in the environment of other people - you're just firing out a lot more infected particles," he explained.
"It means you can infect a lot more people in the same period of time or you can infect people with very fleeting contact, and that's why the Delta variant is taking over and why evolution is selecting it to be the dominant one, because it's making far more copies of itself."
Prof Baker said if Delta is in the community, Kiwis will need to get used to wearing masks, because it's the best protection against transmission.
"The big message is that we need to recognise that this is a virus that's transmitted through the air. It's not really through contaminated surfaces," he said.
"It's good to wash your hands and use hand sanitizer, but the big thing we need to be doing is wearing masks so we're not firing out these particles, and also it gives you some protection if someone else in your environment is breathing out these particles.
"If we don't want to have to just go home and stay home to avoid the virus, if we want to actually share public transport or go to school and work when there's a heightened risk, we need to be very comfortable with wearing masks in those situations."
Prof Baker acknowledged how worrying it will be for people to learn of a community case in New Zealand, particularly since New South Wales is currently battling its worst outbreak ever after Delta spread through Sydney and beyond.
The Australian state reported 452 community cases on Tuesday and one death, after eight deaths and 478 cases were announced on Monday.
"We've had a very good run in New Zealand for five months without any cases in the community. We've always known that, based on past experience, we would expect to see more outbreaks," Prof Baker said.
"We can be fairly certain that this will be the Delta variant. That's absolutely the dominant strain of the virus at present. But even though we're expecting this to happen at some point, it's always a shock to actually have that confirmed, that we do have a case in the community.
"No doubt the contact tracing will be happening very rapidly and that will help to identify an obvious source. If there isn't an obvious source, we would expect a very intense lockdown I think, at this stage."
The Auckland Regional Public Health unit is undertaking interviews with the case for contact tracing purposes. The case is located in Auckland and a link between the case and the border or managed isolation is yet to be established.
The announcement of the community case comes just hours after an investigation found that COVID-19 transmission at Auckland's Jet Park quarantine facility was likely caused by room doors being opened simultaneously for just seconds at the same time.
Joint head of managed isolation and quarantine (MIQ) Brigadier Rose King released the findings on Tuesday of a probe into the origins of three COVID-19 cases detected in the Jet Park facility between July 27 and July 29.
The Ministry of Health's Deputy Director of Public Health Dr Harriette Carr said the investigation by Auckland Regional Public Health Service (ARPHS) found that while there were no bubble breaches, the three cases were genomically linked to a person in the room opposite.
ARPHS found that doors to rooms on opposite sides of the corridor were opened at the same time for about three to five seconds on four occasions when a COVID-19 case in the room opposite would have been considered infectious.
"The results of the investigation of infections at the Jet Park hotel has just confirmed what we already know about this virus, that it's very transmissible as an aerosol, and that means that it's not just a matter of droplets which have a limited range," Prof Baker said.
"Aerosols can waft in a stream of air for a considerable distance, and even after someone has left the room or left the corridor, they're still capable of infecting people."
Almost six months free of community transmission
The new case of COVID-19 breaks New Zealand's almost six-month period free of any community cases. The last community cases were reported in February, linked to Papatoetoe High School when Auckland was thrust into alert level 3 lockdown.
COVID-19 Response Minister Chris Hipkins issued a stark warning earlier this month, that the Government wouldn't hesitate to implement an alert level 4 lockdown if the Delta variant of COVID-19 emerged in the community.
"The main variant is the Delta variant - it's more transmissible, causing more hospitalisations and more deaths and making younger people sick," he told reporters.
Hipkins said if the Delta variant appeared in the community - as it has in Australia with devastating results - the Government would act swiftly and impose alert level 4 restrictions.
"In the event we discovered Delta we would move to alert level 4... We maintain the best economic response is a strong public health response, and a short sharp lockdown is likely to be more successful."
Hipkins warned any future lockdowns would be announced with very little notice and urged New Zealanders to have a plan.
"My message to all New Zealanders is this is not over we are still going to be dealing with this for some time to come."
Prime Minister Jacinda Ardern will provide an update at 6pm on the Government's response to the latest coronavirus case.