A 4-year-old boy has died of flu in what experts are warning is one of the most intense seasons in recent years.
The Government has extended the free vaccine programme until September 11 and Health Minister Jonathan Coleman warns the numbers of sick could get worse in coming weeks.
Specialists say immunisation is the only way to guard against getting sick.
It is not known whether the 4-year-old Auckland boy was vaccinated, but the vaccine protects against the two strains of flu prevalent in New Zealand.
He was admitted to hospital suffering pneumonia but died in mid-July.
The youngster is one of two people in Auckland killed by the disease, which worst affects the young and old. The other victim was a 77-year-old woman.
It is the same H3N2 Influenza A strain that claimed one life and left many needing hospital treatment in 2012, according to the Shivers Project - a five-year flu monitoring programme that is part of a global plan to fight the virus.
And although the number of people with flu symptoms visiting their doctor dropped last week, Ministry of Health's director of public health Dr Stewart Jessamine said the worst may not be over.
"Case numbers may continue to grow. Typically every two to three years there are an increased number of influenza cases in the community."
Winter-related illnesses have been stretching hospital services, with health boards cancelling elective surgery and urging the sick to visit their doctor rather than clogging emergency departments.
More than four times the number of people have been infected this winter compared to last year.
Coleman said: "The latest data from ESR shows 124 cases per 100,000 population, compared with 26 cases per 100,000 at the same time last year."
Dr Sue Huang, who heads the Shivers study in Auckland, said most children had immunity to fight the virus and the death of the 4-year-old boy was a tragedy for the family.
The number of patients being treated in Auckland's ICUs for severe acute respiratory illnesses nearly doubled this winter - from 45 in the second week of August last year to 80.
"I think it reflects the intensity of the flu activity this year - it's higher than other years," said Huang, who is also the World Health Organisation's National Influenza Centre director.
Huang said the Shivers Project showed a pattern with those who were under 4 or those over 65 years the most vulnerable.
"Other viruses can kill people as well but flu is a very big issue for people in those age groups and those with have underlying conditions."
Middlemore Hospital head of emergency medicine Vanessa Thornton said a long and sustained flu season was worsened by a cold winter coupled with people in poor housing with inadequate heating living close together spreading disease.