The chilly weather appears to be responsible for a sudden rise in the number of New Zealanders with flu.
The number of people going to the doctor with flu-like symptoms has almost doubled in the past month to 58 cases in every 100,000 people, the National Influenza Strategy Group said in a statement.
Virologist Lance Jennings said influenza was a serious disease, especially for people with underlying medical conditions. It could make their condition much worse and lead to hospitalisation or even death. Dr Jennings said a free flu vaccination was still available for eligible New Zealanders - pregnant women, people aged 65 and over and anyone with long-term health conditions - until July 31.
"Contrary to a widely-held myth, you cannot get influenza from the vaccine, as it does not contain any live virus. Unfortunately some people may be incubating a common cold when vaccinated and then develop respiratory symptoms due to a non-influenza virus.
"Around a million people have already been vaccinated but that still leaves many vulnerable people unprotected.
"People, especially those in at-risk groups, should be immunised as soon as possible. Influenza cases traditionally begin to rise sharply at this time of year and it takes up to two weeks to develop full protection after vaccination.''
Influenza was usually characterised by a sudden onset of illness, high fever, headache, a dry cough and usually lasted seven-10 days.