Two flu viruses are circulating throughout the country, and the number of people getting them is higher than at the same time last year.
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The A-strain is being reported in greater numbers in the North Island and affects the elderly and the very young more severely than other strains. It is covered by the flu vaccine.
The B-strain is more prevalent in the South Island.
National Influenza Specialist Group spokesperson Lance Jennings said it was unusual for two viruses to be circulating, and it was possible a person could contract both.
"It is entirely possible but the seasonal influenza vaccine that is currently available should provide good protection against both the A- and B-strains that are circulating in New Zealand."
Listen to Lance Jennings
Health Minister Jonathan Coleman said today free access to the vaccine for those most at risk would be extended from the end of July to 31 August.
The vaccine was delayed worldwide by a few weeks this year, but despite that about 1.2 million doses have been distributed nationwide, protecting about a quarter of the population.
Mr Coleman said health officials believed influenza had not yet peaked this winter.
Immunisation Advisory Centre director Nikki Turner urged people to get themselves vaccinated - particularly those over 65, pregnant women, people with long-term health conditions and children under five who have had respiratory illnesses.
"A[-strain] looks like it's the predominant but both of them it looks like we are seeing in many parts of the country now, so we are definitely now well and truly into the flu season," she said.
Dr Turner said the vaccine was not perfect but offered better protection than doing nothing.
"Flu can be life-threatening... For most people, it just gives you a really lousy bug where you need to go to bed and feel lousy for a week.
"But there are some people who go on and get pneumonia and certainly people with other underlying conditions can die from it."