Flu vaccinations will be available a month later than usual this year, because the vaccine formula has had to be strengthened.
The vaccine is usually out in late February, but will now be on doctors' shelves at the end of March.
A particular strain of the virus has proved more troublesome in the northern hemisphere than doctors expected.
It was not caught in time for an improved vaccine to be ready for the northern winter.
Dr Lance Jennings from the National Influenza Specialist Group, which runs the flu vaccine programme in New Zealand for the Ministry of Health said its effectiveness would not be known until the flu season hits.
"We have a good guess at what viruses are likely to be circulating, we can look at last year's northern hemisphere season and see how they fared and make some guesses, but the end of the day, the best protection is to receive a seasonal influenza vaccine, on an annual basis."
Dr Jennings said the delay in getting the vaccine shouldn't affect the number of people who get the flu.
The United States Centers for Disease Control and Prevention said the usual vaccine was only about 23 percent effective, compared with between 50 and 60 percent in the past.
Health officials picked up the problem early though, and added protections for the so-called Switzerland strain to the southern hemisphere's vaccine.