The number of confirmed swine flu cases in New Zealand spiked to 100 on Tuesday, triple the number of cases reported last week, as the virus started to spread through the community, officials said.
Some schools have closed or sent large numbers of pupils home as the influenza A (H1NI) virus spreads.
Deputy director of public health Fran McGrath said authorities were still trying to contain the spread of swine flu, so it did not peak at the same time as the normal southern hemisphere winter outbreak of seasonal flu.
The symptoms of swine flu were normally mild and most individuals would not be badly affected by the disease, she said.
"But it's the impact of widespread absences, business continuity, schools and workplaces, that is likely to be the severe impact," McGrath told Radio New Zealand.
Health Minister Tony Ryall said Monday that a widespread outbreak would strain hospitals throughout the country.
"Normally we would see about 31,000 medical admissions to hospitals a month during winter. Officials think we could end up with another 4,000 or so if the swine flu peak matches the winter peak," he said.
Until the weekend, all the cases reported had been among people who had travelled overseas or their close contacts but some of the new cases indicate the virus is starting to spread further.
Swine flu, which was first detected in Mexico in April, has so far infected almost 36,000 people in 76 countries and claimed 163 lives, according to the latest World Health Organization figures.