Outbreaks of H1N1 swine flu have occurred this summer in Wellington, Geraldine, Canterbury and most recently Hawke's Bay.
Dr Lance Jennings, who's a virologist with the Canterbury District Health Board, said it's unusual to see so many people get sick in the warmer months.
"I personally haven't seen this type of activity since 1974 in New Zealand. Australia is reporting a similar phenomena, in Queensland they've had double the number of hospital admissions due to influenza."
He said cases of the disease are being tracked, but the root cause is unclear.
"Whether it's associated with the weather, whether it's associated with tourists introducing the virus from North America or South East Asia we just don't know."
The latest outbreak is in Hawke's Bay, where eight people have been treated for swine flu so far this month.
One woman is still being treated at Hawke's Bay Hospital's intensive care unit - her sister Nikki says she first got sick 20 days ago.
"I realised she was ill and she was getting worse but I had no idea swine flu was even still around."
She says the experience has been terrifying and urges others to get vaccinated.
"You're just sitting with someone who's totally unresponsive and got lots of tubes and wires, just waiting."
Dr Jennings said the best defence for influenza is vaccination.