Half of the patients in Wellington Hospital's Intensive Care Unit (ICU) have influenza, disrupting and delaying operations.
Eight of the 16 patients, who range from young children to people in their 60s, are infected with the potentially life-threatening virus.
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Specialist Dr Paul Young told RadioLIVE the numbers are unprecedented.
"In the time I've been a specialist I've never seen the proportion of patients so high."
The demand on the ICU is disrupting elective surgeries and means hospital staff have had to delay some operations.
"It means we have less beds available to allow us to do complicated operations like heart surgery," Dr Young says.
He says the week's sudden spike is likely due to a delayed flu season, and emphasises the importance of getting the flu shot each winter.
"Influenza can be a severe and life threatening infection even if they are healthy, and it's a good idea to get immunized.
"It's the best defence we have available and certainly all of us in the Intensive Care Unit here get immunised."
Newshub.