The illness that closed a Westport primary school this week has been identified as influenza, the West Coast District Health Board says.

Westport North School was forced to shut its doors on Tuesday after 66 students - one third of its roll - fell ill with headaches, nausea, fevers and vomitting.

Samples taken from the school have returned positive for influenza.

DHB Medical Officer of Health Ramon Pink said those who had been sick should stay home from school or work for at least five days after first experiencing symptoms.

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He also urged people to call a GP for medical advice rather than visiting in person.

The school is set to reopen tomorrow after cleaning and disinfecting all surfaces, but Dr Pink said the number of absentees was likely to be high for some time.

Westport North principal Cath O'Loughlin previously said she was also not expecting all students to be back by then.

"I know that when the public health nurse was meeting with these families, the children that she saw were fairly sick and the families intimated they wouldn't be back on Friday when we reopen."

- RNZ