The Covid-19 community outbreak is stretching the limits of the country's quarantine system, and the number of rooms available with suitable air filtration systems that can withhold the virus.
There are now more positive Delta cases in the community than the government has rooms available with top-of-the-line ventilation systems.
A public health expert says the government has dropped the ball with the state of its quarantine facilities.
As of Wednesday, there were 687 positive cases of Covid-19 in the community, most of which had been moved into quarantine facilities in Auckland.
Joint head of managed isolation and quarantine Brigadier Rose King said there were a total of 674 quarantine rooms that have air filtration units installed.
While not every person who had been infected would have their own individual room, the outbreak was reaching a point where the number of suitable rooms were being stretched.
University of Otago public health expert Professor Nick Wilson said that figure was not good enough at this point of the pandemic.
"That number of filters does seem too low from what I know of these hotels."
Professor Wilson has long-standing concerns about the repurposing of hotels into quarantine facilities, and said more of the filtration systems should have been installed.
Covid 19 Response Minister Chris Hipkins said the government's current quarantine options were due to fill up in the next 24 hours or so.
But another hotel is joining the quarantine queue.
"We have got an additional facility that's being stood up at the moment that will bring several hundred additional rooms - that's the Holiday Inn out by the [Auckland] Airport," Hipkins said.
"It's been converted for use for quarantine purposes. That will bring several hundred more rooms in.
"We will need that extra facility coming on in the next 24 to 36 hours in order to make sure we can accommodate extra cases tomorrow and the day after and so on."
Not part of original plan
Brigadier King said installing air filtration units in returnee and now quarantine rooms at places like the Holiday Inn was not part of the original plan with MIQ facilities.
"The priority remains improving the ventilation of the shared spaces across our facilities, primarily corridors and lifts," King said.
"But as an added layer of protection work is underway to install air filtration units in quarantine rooms across our facilities."
University of Auckland aerosol chemist Dr Joel Rindelaub said efforts to add things like HEPA filters was good news.
"HEPA filters, these High Efficiency Particulate Air filters, are pretty much the best thing that we can use when we're recircling the air to keep it clean, so that's a good sign to see," Rindelaub said.
"I'm also glad that they're taking an interest in these hallways because this is the exact spot where we know that aerosol transmission events have taken place previously here in New Zealand."
Professor Nick Wilson cautioned, however, for people not to become too reliant on technology such as HEPA filters to keep them safe.
He said the government needed to come up with a new plan for quarantine.
"Having these type of filters is going to help, but we're still in a situation where these modified hotels are not fit for purpose," he said.
"We really, as a country, need to be moving to what Australia is doing and building purpose built facilities which are single storey separate units like they have in Howard Springs in Australia, and all very good ventilation.
According to the Ministry of Business, Innovation and Employment's health experts, the risk presented by ventilation systems at quarantine facilities has been assessed as low.