Tighter restrictions could be deployed to combat the Delta variant as infectious disease experts say lockdown rules need bolstering for the tough new Covid-19 strain.
Those rules might require staff in large indoor workplaces such as supermarkets to wear N95 respirator face masks and be vaccinated.
Prime Minister Jacinda Ardern today said the risk of Covid-19 spreading in workplaces, including those deemed essential, might require stricter rules.
"If we need to tighten up our restrictions further we will," Ardern said.
Dr Siouxsie Wiles said that when keeping windows or doors open was impractical, or when ventilation cannot be retrofitted, vaccines and N95 face masks might be necessary.
Physical distancing, perspex barriers and even low-grade face masks weren't good enough at stopping Covid-19 in some enclosed spaces, the microbiologist said.
The origin of some new Covid-19 cases was not yet determined, but most are linked to households and workplaces.
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Sistema Plastics has had to close its factory in South Auckland's Māngere until Wednesday after an employee tested positive for Covid-19.
Elsewhere, hundreds of New Zealand's essential workers from the retail sector are in isolation.
Knowledge of how Covid-19 transmitted had changed since last year's national lockdown, and the more infectious Delta strain changed the situation even more, Wiles said.
"The protocols we have are perhaps a bit of a leftover from the first lockdown," the University of Auckland Associate Professor said.
"It's hard to retrofit proper ventilation to places. There will be places where it's not practical to have windows and doors open."
This could include food preparation essential workplaces, Wiles said.
Countdown on Thursday said more than 1000 employees were isolating, for reasons including visits from customers who tested positive for Covid-19.
An MIQ worker at Four Points Sheraton Hotel in Auckland tested positive for Covid-19. Photo / Alex Burton
Wiles said N95 or similar-grade masks were generally more effective at filtering out airborne particles than many generic models.
Some healthcare workers this year have complained of N95 mask shortages.
Middlemore Hospital this week said N95 or P2 respirators were mandatory for Emergency Department staff and for staff on wards where Covid-19 patients were admitted.
Hospital staff performing initial screening of admissions must wear N95 respirators as well, and Middlemore said physical distancing rules applied for any staff who removed masks to answer phone calls.
Middlemore explained the rules after a Herald reader who learned of a recent Covid-19 case at the hospital claimed not all staff on site were wearing masks early last week.
Wiles and epidemiologist Professor Michael Baker both wondered if New Zealand's current essential worker category was too broad.
Baker said there might be areas where essential industry support staff working on site could shift to working from home.
Baker said indoor environments generally posed the greatest risk of spreading Covid-19.
The largest sub-cluster in the current cluster has been linked to a major church event in Māngere.
Baker has frequently called for discussion about mandatory mask use at different alert levels, especially in indoor public places and on public transport.
He said last year's alert level system was assembled adroitly at short notice, adapted from a Singapore model, but the pandemic had changed, as had knowledge of it.
The University of Otago public health expert said the two most severe tiers were both effectively lockdown, but alert levels 1 and 2 were effectively nothing.
Baker said aside from any changes to level 4, lower alert levels might have to be tweaked, such as with additional face mask mandates.
He did not voice support for more restrictive conditions such as curfews but said consistent, possibly enhanced enforcement of current rules was needed, especially around travel.
In Sydney, curfews were imposed on western suburbs last weekend but New South Wales recorded a record-high number of new Covid-19 infections today, with 1218 cases.
Baker said data from any mystery cases, and further details about how Delta was transmitting, should inform the next steps in any alert level rule changes.
He said some hope could be taken from the fact most current cases had known links and were not mystery infections.
Efforts to contain the formidable Delta strain now occupied more than 1700 contact tracers, Director-General of Health Dr Ashley Bloomfield said.
Earlier today, physicist and Covid-19 modeller Professor Shaun Hendy said the number of infected essential workers was critical to understanding the outbreak's trajectory.
"The big concern is we get an outbreak in that network of essential workers, obviously across supermarket chains for instance."
Data on the extent and origin of essential worker Covid-19 cases would need to be monitored for days to come, Hendy said.
