"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.