Ministry of Health officials are reviewing the current advice around masks and the most effective ones as it is confirmed the highly transmissible Omicron variant is spreading in the community.

In a conference on Sunday afternoon, in which the prime minister announced the whole country would move to the red setting of the Covid-19 framework, Jacinda Ardern said an assessment is underway on mask advice as evidence emerges from other countries dealing with outbreaks.

"There is evidence emerging around those masks that offer better protection than others, including the effectiveness of surgical [masks], so not just N95s."

She says a formal update will be provided in the coming days.

Director-General of Health Ashley Bloomfield says they have been reviewing the updated evidence around medical/surgical masks and also N95s.

Brisbane-based data scientist Jeremy Howard, who in 2020 did the world's largest review of evidence on face masks and the ability to stop aerosol spread of Covid-19, told The World Today that the evidence is evolving.

"Ditch your cloth masks and also ditch your surgical masks," he says.

"Omicron is so aerosolised, so it's such tiny particles that float in the air.

"When you breathe in, you're sucking in those respiratory particles."

However, Bloomfield says N95 masks need to be properly fitted to be effective.

"All our workforces who are using those masks, in the healthcare system and at the border, have proper fit testing, because if they're not fitted properly then they can be less effective than a normal cloth or indeed a surgical/medical mask.

"So we're providing advice, which will be nice and clear for the public but also through to the government, about access to masks and just how people can make sure they can access the masks and the timing of the use of those."

University of Auckland associate professor and microbiologist Siouxsie Wiles says people could be infectious before realising it, which means wearing a good quality mask is important.

"One that fits well and covers your mouth and nose. If you are only wearing a fabric mask, consider upgrading to one that includes a PM2.5 filter or wearing one over a surgical mask.

"FFP2, Kn95, or KF94 masks are good options but beware of where you buy these from as there are many counterfeit masks on the market.

"Masks can be reused, so try to have one for each day of the week, and store in a paper bag between uses. Replace if they become wet or soiled, or the straps loosen so they don't fit as well."

Indoor environments where people don't wear masks are high risk for transmission of the virus, Wiles says.

"This is why I am concerned about hospitality and other similar venues."

Under the 'red' setting, hospitality venues, such as cafes, bars, restaurants (excluding takeaway-only businesses) and nightclubs, operating with vaccine pass requirements can have up to 100 people, seated, and spaced at least one metre apart.

If a hospitality business chooses not to follow vaccine pass requirements, it can open for takeaway only and should follow the rules for retail.

But Wiles says: "It doesn't matter if groups of people are seated one metre away from each other, if they are indoors, especially if the place is badly ventilated, then the risk of transmission is high and we've seen lots of examples of Omicron spreading in these types of settings overseas. Improving ventilation and introducing air-purifiers are ways to reduce this risk."

Epidemiologist Amanda Kvalsvig, from University of Otago in Wellington, says while the overall approach to reducing the spread of Omicron is "highly appropriate", there are still gaps that need to be addressed.

"The government needs to move away from its current stance on respirator masks (eg, P2 or N95) which appears to be that the public would not understand how to wear them.

"This stance would deny New Zealanders one of the most effective protections that they have just now.

"Respirator masks are standard wear in many countries and there is abundant clear and straightforward advice about their use."

Dr Kvalsvig says even according to the United States' public health agency, N95 masks tend to fit better around the face than surgical masks, "which typically bunch up at the sides so they have gaps for the virus to enter".

"The prime minister has signalled that lockdowns will not be used. If that is the case and New Zealanders will be mixing in public while Omicron is circulating, then people must have access to effective masks. That is particularly true for children who are the least-vaccinated age group.

"This protection should not be available only to more privileged members of society who are already ordering masks from overseas."

On 14 January, the US Centers for Disease Control and Prevention updated its advice on mask use, saying people could choose to wear N95s and KN95s, but that people should still wear "the most protective mask" that would fit them well.

Dr Kvalsvig says the New Zealand government should consider importing enough masks so that everyone had at least one to use or a set they could rotate every five to seven days.

"While universal mask access is being arranged, priority should go to supplying effective masks to those most at risk, including essential workers."

Ardern says the health workforce has a good supply of N95 masks.

-RNZ/ ABC