A sharp spike in flu cases has come late in the season this year, adding to the pressure the health system is under already.
By Kate Green for RNZ
Dr Hannah Cooper, a public health physician in the health intelligence team at PHF Science, told RNZ there were a few different ways of defining a "bad" flu season.
This season was bad because a lot of people were getting flu at once - but there was no conclusive data yet to show this particular strain was making people sicker than in previous years.
What's making this flu season so bad?
Officially, flu season in New Zealand runs from May to October.
The virus causes symptoms like a fever, runny nose, cough or upset stomach, and there are many different types or strains circulating each year.
Each flu season is influenced by the one before. Last year, we saw the emergence of a strain of flu known as "Super-K".
It peaked late, causing an extended flu season that means people have retained some immunity from last year's infection further into this year.
But because the season has peaked later in the year, the immunity boost those of us who got the flu vaccine gained at the start of winter has worn off a bit, leaving us more vulnerable.
Cold weather can't give you the flu, but it can make it spread faster.
According to Cooper, the cold snap felt around the country in early August would have increased transmission.
That's around the point when ambulance callouts began peaking, too, to reach an all-time high the week of 10 August.
Viruses spread more easily in cold weather, as low temperatures and dry air help viruses survive longer, while weakening your body's natural defenses at the same time.
People also spend more time inside when it's cold, sharing poorly ventilated air, which increases the chance of infection.
This, plus patchy vaccine uptake, aided transmission in the community.
As of 6 September, 1,256,766 people had received the flu vaccine - less than a quarter of the population - most of them in May.
Even though the H3 subclade, the dominant strain this year, was around last year, New Zealanders still seemed broadly susceptible to it, Cooper said.
That said, the vaccine appeared to be a good match. Cooper stressed flu vaccines were better at reducing serious illness than preventing infection, so the high numbers of flu didn't necessarily mean this vaccine wasn't as effective.
So far, the data showed the dominant strain of flu appearing in hospitals was one that was covered by the vaccine.
How does this flu season stack up compared to those in the past?
That question, Cooper said, was very hard to answer.
Flu cases had been tallied using different methods over the years.
These days, it was done by the SARI method - that is, those presenting with Severe Acute Respiratory Infection - in the Auckland region, as a representative snapshot of the wider population.
"I don't think that anyone would be able to say how this year compares to say 20, 30 years ago."
The country's population dynamic was also changing over time, in terms of size and co-morbidities and age structure. That meant the impact of a bad flu season in terms of pressure on hospitals in 2026 was probably quite different than it might have been in previous decades, Cooper said.
How much pressure is it putting on hospitals?
The effect of a large number of flu cases at once is hitting harder this year because the health system is already overwhelmed.
Health NZ put on more staff to ease pressure of the winter season, but clinicians in EDs in particular have said they're still overwhelmed by demand.
RNZ has previously reported hospitals were overflowing well before flu cases reached their peak, according to Health NZ data.
The occupancy rates of eight hospitals at 8am over eight days from 7 to 14 August showed most had more patients than staffed beds.
But the latest data shows a downturn in cases - from a peak rate of 16.3 cases per 100,000 people the week ending 30 August, to 13.7 per 100,000 people the week ending 6 September - which could signal the end is in sight.
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