Prof Plank said the recent rise in cases is most likely being caused by the new BA.5 variant, an Omicron sub-variant first detected earlier this year in South Africa and is on its way to becoming the dominant strain of the virus in Europe and Australia.

"We can see in the sequencing data [BA.5] is spreading more rapidly than the previous variant, which was BA.2. So it's clear that it has a growth advantage, it's able to spread more easily and we're now seeing that filter through into the number of cases by the look of things."

He said BA.5 is "able to evade our immunity to some extent". That means it can infect people more easily and could also reinfect people if they have had BA.2.

"It may be a bit more infectious as well. That's difficult to say but this combination of factors means that it can spread more rapidly."

COVID-19 modeller David Welch told Newshub on Monday that a major rise is on the cards. 

"We might see case numbers up [to] around the 20,000s again. Last time we got to 25,000 and we could easily see those numbers again," he said.

"People should be prepared to go into a higher alert level setting just to take the peak off this wave."

One of the concerning aspects of the recent jump in cases for Prof Plank is the age of those being infected.

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"Back in the March wave, younger age groups were hit very hard. But actually, the number of cases in older groups was much, much lower," he said. "Whereas now, actually, cases in over 70s are higher almost than at any time in the past.

"Even if the number of cases is lower than what we had in March, actually the potential for the health impacts could possibly be on a level with what we have back then because of that number of older people getting infected."

On Tuesday, the Ministry of Health also confirmed the Omicron subvariant BA.2.75 had been found in New Zealand, but there "is no evidence that BA.2.75 requires a shift in public health settings already in place to manage other Omicron variants".

"BA.2.75 is a recently identified second-generation subvariant of BA.2, the dominant variant circulating in New Zealand at this stage. BA.2.75 has only been recently identified as distinct from BA.2 and evidence of its transmissibility, immune evasiveness and severity is still preliminary and emerging.

"We do know BA.2.75 has some characteristics that look like they may enhance its ability to evade immunity, similar to the BA.4 and BA.5 Omicron subvariants and there is some early evidence overseas that it may be slightly more transmissible than BA.2. There is no current evidence that it leads to more severe disease, although assessing the evidence is at a very early stage."

So what does the rise in cases mean for our COVID-19 restrictions?

At orange, there are no gathering limits and face masks are only required in some indoor settings, like in retail and on public transport. In red, however, there are indoor capacity limits of 200 people and masks must be worn at hospitality, which also has special seated and separated rules.

In announcing New Zealand would stay at orange last week, Dr Verrall said new measures were being introduced to try to reduce the spread of COVID-19. That included a second COVID-19 booster and more masks for children in schools. 

The minister said moving to red was "unnecessary" at that point, with orange enough to "manage the virus" alongside the new measures.

"The last time we were in red, we had over 10,000 cases a day. So we're not at that stage. But what I would say is there are things that everyone can do to prevent us from going to red. 

"Wearing masks, particularly indoors, making sure that you're vaccinated for COVID, the booster has been rolled out and making sure that you open a window if you're in an indoor seating."

Prof Plank said on Tuesday that "restrictions are always going to be controversial at this stage in the pandemic", but it was important people understood key messages such as the benefits of working from home if possible, using masks and the fourth dose was now available for some groups. 

"That's really important. The vaccine is still the best tool we have to fight this virus and so having a strong uptake with older people, in particular, getting their fourth dose, as well as people going out for their third dose or even that first dose, if they haven't had it yet, is going to be the most important thing to controlling this wave."

In terms of what might be a trigger point for moving to the red settings, Prof Plank said it's a question for the Government. 

"Obviously it will depend on how the health system is coping. That's not just COVID, that's other pressures on the health system, including flu at the moment. So all of these things are going to need to be taken into account."

Newshub asked the COVID-19 Response Minister this week what could lead to a return to the red traffic light setting. Dr Verrall said Cabinet makes its decisions on traffic light settings based on advice provided by the Ministry of Health.

"Settings under the COVID Protection Framework are regularly reviewed by public health officials. These reviews look at a range of factors - not just case numbers - including scenario modelling, public health considerations and the current capacity of the health sector."

A spokesperson for the Ministry of Health meanwhile said reviews of the traffic light setting would consider "the capacity of the health sector and its workforce, the transmissibility and severity of the COVID-19 variants circulating community, scenario modelling and daily case numbers".

"We are continuing to keep our response to the current community outbreak of COVID-19 under review and will adapt it as the outbreak and pandemic evolve and as part of our resurgence planning.

"Our ongoing management of the response is important for ensuring we protect vulnerable populations, encourage vaccination and promote messages and public health measures that allow people to protect themselves and their loved ones."