Covid-19 modeller Professor Michael Plank said one big contributor was the rise of the faster-spreading Omicron subtype BA.5.
"Last week, we were seeing case increases of about 40 per cent from one week to the next, which is a doubling time of about 14 days."
In the Tairawhiti region – long New Zealand's least-infected area – the seven-day average case rate had fallen to the 30s before numbers picked up again last week.
The 96 cases Hauora Tairāwhiti reported yesterday was the highest seen in one day since April.
"Part of the reason for the rise may be the emergence of the BA.5 subvariant of Omicron," Te Whatu Ora Health New Zealand –Tairāwhiti district director Jim Green said.
"This is increasing nationally and expected to be dominant by mid-July. We only have a few strains with genomes analysed, but for the last week of June, seven of 11 genomes analysed from our district were BA.5."
For the first time, authorities have also recorded the arrival of BA.2.75 - a recently-identified second-generation subvariant of BA.2.
While evidence on its transmissibility, immune evasiveness and severity is still emerging, the Ministry of Health advises there's nothing to suggest it requires a shift in public health settings.
Plank, of Covid-19 Modelling Aotearoa, has said a national peak of more than 20,000 daily cases is possible, similar to what was observed in March.
As we couldn't yet be sure the rapid growth trajectory would continue, it was hard to tell just how soon that peak might arrive - or as soon as this month, as another expert has suggested possible.
What was clearer, however, was the uniform geographical pattern under which cases were increasing.
"It's taking off across the board and most DHBs have seen a marked increase in cases over the last week or so," Plank said.
"I think the border reopening is probably part of it. Back in February, when the Omicron wave was taking off, we still had pretty strict border controls and so the number of seed cases coming in was quite small.
"One or two cases led to that outbreak taking off and gave Auckland a head-start, just because that's where those border cases happened to land.
"Whereas now with our borders open, we've got a larger number of seed cases coming in and potentially starting parallel outbreaks in different parts of the country."
Otago University epidemiologist Professor Michael Baker agreed.
"The whole country is being seeded quite liberally with these new variants because people are flying in and going to all sorts of places around New Zealand – which may be the single biggest driving factor."
Otago University virologist Dr Jemma Geoghegan said there'd been around 5000 arrivals into the country since borders re-opened - and this equated to about one internationally-introduced case each day.
"That's not including anyone that tests positive after arriving from overseas - these are people who've tested positive but haven't been overseas, and their [virus] genome is related to recent arrivals," she said.
"That's a lot of introductions."
When borders fully reopened to international tourists at the end of this month, she expected these rates increase further.
More broadly, Baker said there were no longer any controls in place – particularly in schools – as there were in the first Omicron wave to curb the virus, at a time seasonal factors were coming to bear as well.
"There is now an equilibrium between the virus, and us and our immunity."
Plank said similar attack rates being observed across New Zealand also suggested our immune profile was similar from region to region.
Nearly 2.6 million Kiwis happened to have received their booster more than 90 days ago – and it was known booster effectiveness against symptomatic disease could fall by half over 90 to 180 days.
A second booster dose, which last week became available for at-risk people who'd had their first booster more than six months ago, would help restore immunity among the vulnerable, Plank said.
"If it's been six months since their third dose, then now is a good time for them to get another dose and really boost their immunity back up."
That was all the more important given cases among people over 70 were now the highest they'd ever been.
"Back in March, cases were predominantly among younger people," Plank said.
"So that shift in age breakdown is a cause for concern, because even if the number of cases is significantly lower than it was in March, the health impacts could potentially still be on par."
Added to that, he pointed out the health system was already "incredibly busy" grappling with Covid-19, flu and other resurgent seasonal nasties.
For now, the Government is keeping New Zealand at the orange traffic light setting - but some experts have suggested a return to red might soon be warranted.
