Covid 19 Omicron outbreak: Jacinda Ardern deflects speculation NZ will return to red traffic light after 3000-plus spike in cases

Prime Minister Jacinda Ardern has assured there is "no expectation" the country will return to red traffic light restrictions despite a single day spike of 3000-plus Covid cases that has experts and teachers alarmed.

New Zealand registered 9629 new community Covid cases yesterday - a significant jump on Monday's 6498 cases that epidemiologists say is being driven by the emergence of the new Omicron BA.5 subvariant.

Professor Michael Baker has described the increase represents a "very distinct" upward trend in cases over the past week, but also placed yesterday's dramatic single-day jump in context.

"Case numbers are always big on a Tuesday because it picks up on the weekend tests – that pattern has been there for months and that's why the moving average is so important," Baker said.

"All the signs point towards a large wave of Covid-19 with an abrupt rise in cases."

Yesterday's seven-day rolling average of Covid cases of 7246 has jumped by 1766 cases compared to the same time last week.

University of Otago Professor Michael Baker. Photo / Luke Pilkinton-Ching

University of Auckland senior lecturer in computational evolution Dr David Welch said the Government must be considering the possibility of moving the country to the red traffic light.

"Going back to red is something that the Government would be looking at very closely right now, they'd be looking at how the hospitals are coping and what are the projections," Welch said.

"Our hospitals are already really, really full. Not only because of Covid but also because of other winter illnesses. We expect these cases to continue to rise and there'll be just more pressure on the health system.

"I certainly won't rule out going back into red within the next few weeks, but then again I'm not sure if it is politically feasible and whether people will listen this time."

Speaking in Melbourne yesterday, Ardern had to deflect speculation the country could return to the red traffic light - which requires masks in most indoor locations and restricts indoor gatherings to 200 people.

"We have no expectation at this stage. We've recently done a review and decided to stay in the settings we have. But keep in mind we have really important rules at the orange setting that are important to protect us," Ardern said.

"We have to do things that we know will make a difference to what we are seeing now and we know that what makes a difference is mask use and vaccinations. Again, these are restrictions that are not widely used outside New Zealand. We have hung onto them for two reasons."

Prime Minister Jacinda Ardern meets Victoria Premier Daniel Andrews in Melbourne. Photo / Jenee Tibshraeny

Ardern also cast doubt over whether the harsher gathering limits enforced under the red setting would make a marked difference to case numbers.

"There is a real question mark over that, particularly as we are seeing those rates in some of our older New Zealanders. We know the biggest thing we can do to make a difference right now is mask use and vaccines, so that's what we are going to do," she said.

A second vaccination booster shot is now available for all Kiwis over 50. Health, aged care and disability workers 30 years and over, and immunocompromised people are also eligible.

"Having just come from Europe, and also recently the United States and even here in Australia, I can tell you New Zealand is still using measures many other countries don't," Ardern said.

There were 24 Covid-related deaths yesterday and 493 hospitalisations - 11 in ICU. The seven-day rolling average for hospitalisations was 436 - a significant increase on a week earlier when it was 338.

After Omicron first began spreading across Auckland in late summer, the region saw an early-March peak that preceded a "Mexican wave" of cases throughout the country – and some DHB areas didn't record their highest daily counts until a month later.

This July, however, cases across regions are climbing roughly in step – and quickly.

Covid-19 modeller Professor Michael Plank. Photo / Supplied

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."

Modelling experts Plank and Welch both said a national peak of more than 20,000 daily cases is possible, similar to what was observed in March.

Plank said the border reopening was "probably part" of the accelerated arrival of new variants across the globe. On Friday afternoon, analysis of whole genome sequencing confirmed two cases in New Zealand with BA.2.75 - recently travelled from India.

The sooner than expected Covid "winter wave", compounded by a particularly bad flu season, is also having a hugely disruptive impact on schools.

Some schools across New Zealand have a quarter of teachers off sick or isolating, and with few spare relief teachers, some schools have been rostering year levels home.

Dr David Welch, senior lecturer at the University of Auckland's Centre for Computational Evolution and School of Computer Science. Photo / Jed Bradley

Auckland's Carmel College reverted to online learning for the rest of this week with high case numbers among staff.

"Towards the end of last week, we were getting to the stage where we were averaging about 20 per cent of our students away and 25 per cent of our staff away," principal Chris Allen said.

Associate Education Minister Jan Tinetti said they were "mindful that Covid-19 is having an extended impact on teaching, learning and assessment this year" but it was quite different to previous years.

"The Ministry and NZQA are looking at a range of potential measures to support teaching, learning and assessment, including the interventions which have been used in previous years," Tinetti said.

Auckland Secondary Schools' Principals' Association president Greg Pierce expected schools wouldn't hear until September whether measures like extra credits would be on offer.

Following lockdowns in 2020 and 2021, thresholds to get university entrance and merit or excellence certificate endorsements were lowered, and students could earn bonus learning recognition credits depending how badly their region was hit by Covid.