The flu season could last until the end of August in a worst case scenario, according to Counties Manukau District Health Board.
Hospital emergency departments around the country have been flooded with patients as a result of winter illnesses, including influenza, rhinovirus and enterovirus.
The DHB's director of population health Dr Gary Jackson said the normal flu season, which included a range of winter respiratory illnesses, normally only lasted for about four weeks.
But because of Covid-19, the country's closed borders and lockdowns, such viruses had been suppressed and it might last longer this year as a result.
"People don't have the same level of immunity, so it might extend out to eight or 12 weeks," he said.
Jackson said in a worst case scenario it could continue until the end of August.
"It's looking like a cold hard winter at this stage."
Middlemore Hospital has this month had to deal with a spike in patient numbers, with up to 400 people a day visiting its emergency department. The DHB has attributed the surge in patients to winter viruses and the impact of staff taking time off due to Covid-19.
The government announced on Monday it had widened access to free flu vaccines for an extra 800,000 New Zealanders, with children aged three to 12 years and people with serious mental health or addiction needs now eligible.
"We've already seen more than one million New Zealanders get a flu shot, but with significant pressure on our health system we're ramping up efforts to get as many people vaccinated as possible," Health Minister Andrew Little said.
"In recent weeks we have seen a number of pre-school children hospitalised with the flu. We also know children can be transmitters of the flu to others in their family who are more at risk, so it makes sense to reduce transmission as much as we can."
Dr Nikki Turner, the medical director of the University of Auckland's Immunisation Advisory Centre, said this year's flu season came earlier than expected and had been particularly nasty.
"It's much more severe than previous years, so we don't really know when it will peak," Turner said.
"We're asking people to stay home when they are sick and wear masks in public places."
She said while Monday's announcement of more free flu vaccines was good, it would have been more effective if it had been done prior to the start of the flu season.
"But the more people who get vaccinated will help reduce the spread of the flu in the community."
According to the Institute of Environmental Science and Research's flu dashboard, more than three times as many people in recent weeks were self-reporting flu symptoms of a fever and cough than at the same time in 2021. That figure was about 20 times more than in June 2020.
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