"The flu season this year is very different to last year - it has come much earlier and with vastly greater numbers."
ESR public health physician Dr Jill Sherwood said the rise in cases of the flu this year, compared to 2016, was a result of a particular strain that also makes people much sicker than usual.
"We are seeing a lot of a particular Influenza A strain this year, which does tend to make people quite ill and results in more people going to hospital. So you might not have huge numbers [diagnosed] but more of those people are sicker."
Sherwood said Influenza A was particularly dangerous for the elderly who could also get a secondary infection, such as bacterial pneumonia, if they get the flu.
A high proportion of the patients admitted to Auckland City Hospital this flu season, either have other medical problems making them more vulnerable to the flu, are elderly or are in social circumstances that make being sick particularly tough.
There have been about 50 cases of influenza-like illnesses, per 100,000 people, a week since the flu season kicked off on May 1 to the end of July - three and a half times higher than the same period last year.
About half those cases have been diagnosed as the flu.
However, this year's flu rate was still considered mild given the country faced an unusually low season in 2016.
Between 2013 and 2015, flu seasons were moderate meaning there were more than about 80 cases per 100,000 of influenza-like illnesses diagnosed a week.
Sherwood said last year's flu season was so low because there wasn't much change in the flu strain from previous years meaning many people were immune.
"If there hasn't been a lot of change in the influenza viruses that are circulating, more people will have immunity because they were exposed in previous years. Then you've got less people getting the flu...and if less people have the flu there is less of them to spread it to other people as well.
"We are still in flu season and people need to be aware of that and stay home if they are sick, look after themselves, cover their coughs and sneezes to try not spread [the flu], wash their hands if they have been coughing or sneezing into them."
Influenza A causes all the normal flu symptoms including a high fever, body aches, dry cough, sore throat and quite often this year, diarrhoea.
Dotchin from Auckland DHB said although the hospital plans for higher capacity levels at this time of year, the current adult demand is above the usual winter increases, and a significant proportion of this has been flu-related.
"We encourage people to see their GP early, as this could help avoid a hospital admission," she said.
Five tips from the Ministry of Health, to stop the spread of the flu:
1) Wash your hands regularly and for at least 20 seconds, then dry for 20 seconds. Alternatively, use an alcohol-based hand rub.
2) Avoid touching your eyes, nose and mouth.
3) Don't share drinks.
4) Avoid crowded places.
5) Cover your mouth and nose with a tissue when you cough or sneeze.
