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On Sunday, health officials announced another 83 cases in New Zealand, setting a new daily record for this outbreak.
Of those cases, 82 are in Auckland and one is in Wellington, bringing the total number infected in this outbreak to 511.
Dishearteningly, the number needing treatment in hospital rose overnight from 25 to 34.
Ms Ardern called that jump "deeply concerning and the whole reason we've taken the actions we have".
"We know that Delta is more infectious but we are seeing signs telling us it is more dangerous. We are seeing that in our hospitalisations," she said.
Of the 34 hospitalised with Covid-19, two are in a stable condition in ICU.
New Zealand has avoided the number of deaths suffered by much of the world through the pandemic. In the last year, just four people have died from the virus.
The chief reason is New Zealand's elimination strategy, which includes harsh and immediate lockdowns when community cases are identified.
Jacinda Ardern: NZ outbreak at 'critical point'
Ms Ardern's government implemented a level four lockdown - its harshest set of restrictions - within hours of the first case of this outbreak being discovered on August 17.
During a one-hour briefing in Wellington on Sunday, Ms Ardern suggested New Zealand's efforts to execute this strategy hung in the balance.
"It's more important than ever that people should follow the rules ... we're really still at a critical point right now," she said.
Ms Ardern reported three quarters of recent cases were contacts of known cases, and half were household cases.
Director General of Health Ashley Bloomfield said Kiwis should expect "high-ish" case numbers over the next few days.
"It's not necessarily a concern. Where we will be concerned is if we see cases popping up in the community. People should expect numbers will stay up before ... they drop down again," he said.
The high numbers may yet prompt Ms Ardern to impose tougher restrictions.
PM threatens tightening up restrictions
Ms Ardern has business in her sights, saying 25 Kiwis had "exposure events" outside their household bubbles - generally work sites.
"If we need to tighten up our restrictions further, we will," she said.
"We have a small number of workplaces that are operating at level four that have seen transmission within staff.
"This may not be a problem with the rules, say, on the factory floor but what is happening perhaps before and after shifts or even during break time. We're looking at all of this in more detail."
The new rules are likely to apply only to Auckland; the centre of the outbreak with 496 of the 511 cases.
Given the limited spread outside of NZ's biggest city, all regions south of Auckland will move to level three restrictions - which allow for child care, takeaway food and contactless retail - from Wednesday.
Amid the gloom, a bright spot remains in dramatically higher vaccination rates.
Another 78,000 Kiwis received a vaccination on Saturday, meaning almost 14 per cent of the population have been jabbed since this outbreak began.
New Zealand still sits last among OECD nations for vaccines delivered per capita.
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