A Covid-19 modeller is forecasting 200 community cases a day of the coronavirus by Wednesday and 400 daily cases by the end of the week.
Te Pūnaha Matatini modeller Dr Dion O'Neale said because Omicron was in the mix, this week would mark a shift from the previous relatively low case numbers.
"Almost every other place in the world that's had an Omicron outbreak's seen a doubling time of around three days. We'd expect New Zealand to be similar.
"So that means about 100 cases a day at the start of the week, around the middle of the week we're probably looking at around 200 cases a day, and then doubling to around 400-ish by the end of the week.
"Maybe things will go badly - that will arrive on the Thursday or Friday, maybe things will go well and we'll make it to Sunday before we get to that level - it's that rough pattern of taking around three days to double."
O'Neale told Morning Report that even though not all current cases were being confirmed as the Omicron variant, modellers expected it would be the majority soon and were now making their calculations based on its behaviour.
"Omicron cases grow faster than Delta and other variants, so we expect it to out-peak other variants and take over... it's safest for us to be assuming that a case that comes up is Omicron unless we know otherwise."
He warned Omicron had a very fast incubation period, or latent period - the time between a person getting infected and the point they showed symptoms and became infectious themselves.
Once a person realised they were sick, took action to get tested and their results were reported, it could be four or five days from the point of infection.
"That's important to remember: the case numbers we're seeing at the moment, those are infections that are already baked in, that happened some time ago."
This also means that patterns showing comparatively fewer people getting tested for Covid-19 in weekends or on public holidays skew the numbers: "That will affect how those case numbers grow in the short term," he said.
Call to get kids vaccinated on anniversary day
Many children return to school this week, and Northern Regional Health co-ordination centre clinical director Anthony Jordan said that meant the more 5- to 11-year-olds who were vaccinated, the better.
"Even though it's a long weekend here in Auckland - which we're all enjoying - we've got sites opened, our drive through sites, as well as pharmacies and a number of GPs are open to deliver vaccines today," he said.
"The really important thing is to check which site you plan to go to - most of our sites are allowing walk-ins or drive-ins.
"Everyone's welcome," he said - including adults getting a booster shot, dose 1 or 2, or people bringing children to get vaccinated.
Jordan said many parents were hesitant about getting their children vaccinated but health workers are happy to talk to them about any questions they might have.
"A lot of people are a bit more cautious, they want to have a conversation and be assured that this is the right thing to do - we're really encouraging people that have any questions about it to contact their GPs to have a conversation.
"We're pretty comfortable that side effects that we see from people in this group are the sort of things that we expected to see - mild."
Most children getting the vaccination had already had their standard childhood immunisations, and understood that the injection, and any pain from it, was very brief, he said.
"A lot of parents are having their boosters at the same time, with their kids, just to normalise it, to reassure them that this is something okay for them, that's been a useful thing for a lot of kids."
Jordan said the number of children in the 5- to 11-year group who had had the vaccination was good so far, with Auckland DHBs ranging from 33 per cent to 47 per cent vaccinated.
Northland, at 16 per cent, was a bit behind but he was confident mobile clinics covering areas without good access to clinics would help bring that figure up.
"Last week was a big week in general across the [vaccination] programme, I think the arrival of Omicron in the community has put a bit of a focus on getting those vaccinations done sooner."
Vaccination sites were open in both Northland and Auckland on anniversary day.