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About 3300 people visited GPs with flu-like symptoms last week, twice the number for the equivalent period last year.
Environmental Science and Research based the figure on reports of 253 consultations from 75 general practices around the country.
Experts say a relatively small number of people visit doctors. The rest chose to stay home and rest instead.
Gisborne, Wanganui and Northland regions recorded the most cases.
Auckland City Hospital reported that a "significant number" of people needed treatment for flu and other respiratory conditions.
Nationally for the past three years, the middle two weeks of July have been the peak period for the fever, coughs and chesty symptoms that indicate flu.
The number of cases this year isn't as high as in 2005 and 2006, but three strains of the virus were identified last week.
Auckland woman Fatima Avdic fell sick with flu three weeks ago and said it was the worst version of the illness she had ever experienced.
"It was the normal flu symptoms, but multiplied about 20 times," the 36-year-old said.
"Never in my life have I had a flu like that."
Watery eyes, fever, aching muscles, headaches, a cough and exhaustion meant Avdic was off work for a fortnight.
"I couldn't do anything. I went back to work after 10 days, and my boss said I should go home, I wasn't well enough. It was a complete nightmare."
Auckland Regional Public Health medical officer Craig Thornley said numbers in the city usually peaked in the final week of June.
He said the surveillance season runs from May to September, but the number of cases usually started decreasing in August.
Auckland City Hospital general manager of operations Ngaire Buchanan said winter was always busy for any hospital.
She advised anyone who suspected they might have the flu to visit their GP "earlier rather than later".
National Influenza Strategy Group spokeswoman Brenda Saunders said 735,650 doses of the flu vaccine were issued to surgeries this year, around 10,000 fewer than last year.
An estimated 20 per cent of New Zealanders get the flu each year, with the elderly and frail most at risk.
