Auckland Point School is the latest school to be hit by an outbreak of H1N1 influenza with nearly half its 65 pupils away this week.
There are also anecdotal reports of increased cases of swine flu among some businesses and other schools in the Nelson region.
Principal Sonia Hockley said half the children were away from school this week. One pupil and one parent had tested positive, she said.
Testing carried out by the Nelson Marlborough Public Health Service confirmed the illness as H1N1 influenza.
Medical officer of health Dr Jill Sherwood said other family members were now becoming ill from these contacts.
"We are telling the sick ones to stay at home the minute they get normal flu-like symptoms," Ms Hockley said.
She hoped most of the pupils would be back next week as the school was putting on a production. Sunday's school gala would go ahead as normal.
Dr Sherwood said for some people influenza can be a very serious illness.
It was highly infectious and easily passed to other people through coughing and sneezing.
"If you have influenza-like symptoms and are concerned or are getting worse, you should contact your GP as antiviral medicines may help but need to be started early."
Early medical advice is especially important for those with underlying medical conditions such as asthma and diabetes.
Dr Sherwood warned people not to return to school or work until symptoms had died down.
"If you or your family are ill and you are concerned, or if a person's condition worsens, get health advice by calling your GP or Healthline on 0800611116."
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