Health officials are concerned clusters of the harmful H1N1 virus, responsible for the 2009 swine flu pandemic, are appearing unseasonably early.
One woman is critically ill in Hawke's Bay Hospital with the virus and eight others have been admitted to the hospital's intensive care unit.
Dr Nicholas Jones, Hawke's Bay District Health Board medical officer, said the virus had struck before the usual winter flu season, which was cause for concern.
"We don't have an explanation for that. From what I can tell, there hasn't been a big change with it [the virus], which is one of the things you worry about," he said.
"The testing seems to show that it's similar to the one that was around in 2009."