"We've never experienced anything like it."

That's the assessment of Whanganui City College principal Peter Kaua after three-quarters of his staff were struck down with food poisoning, forcing the school's closure today.

Twenty-one of the school's 30 teachers were hit with vomiting and diarrhoea in the early hours of Friday morning.

The staff member in charge of organising relief cover for City College teachers when they're sick received his first call from a teacher at 6.30am. He himself was already sick by this time.

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Mr Kaua said by the time he arrived at school, less than two hours later, 19 staff members had called in sick and the decision was made to close the school for the day. All up, 21 staff members were sick.

"We can cope with about seven staff members being away, but not that many," Mr Kaua said.

Only 10 staff members - including Mr Kaua and the deputy principal, Val Rooderkerk - were unaffected by the illness.

Mr Kaua said the Whanganui District Health Board had been informed of the situation.

He said the staff had been shouted lunch on Wednesday by two teachers' unions, and he thought that was most likely the cause of the illness - especially as the student population appeared to be unaffected.

"We've had a few kids off sick this week, but it's nothing on this scale - just the usual."

Parents were contacted by text message and told to keep their children at home for the day. A message was also posted on the school's Facebook page.

The message said that students who did turn up would be sent home. Parents whose children attended Whanganui City College are asked to contact the school if their child has a stomach upset.

Mr Kaua said it was too soon to say if the school would be open on Monday.

"I'll be making contact with all staff during the weekend and making an assessment then," he said.