Official data shows the past couple of weeks have been the worst in three years for school absences due to illness and medical reasons.
Ministry of Education data shows that last week almost 10 per cent of class time was lost across the country due to illness.
Hawke's Bay has been hit hard; one day last week, Napier Girls' High had 20 per cent of its students, or about 200 girls, off due to sickness. The Terrace School, a primary in Waikipukurau, had 76 kids off today, about a third of its roll, as well as six teachers.
New Zealand Principals' Federation principal Jason Miles told Checkpoint there were more children calling in sick this winter, but schools with ill teachers faced bigger hurdles.
"When you've got a lot of children away, it's fairly straightforward, but if you've got more staff away, percentage-wise, than children, then that's a really difficult space for leaders to be in, because that's about getting enough adults to keep your school open."
Miles said the situation was putting huge pressure on the relieving teacher pool, particularly in high schools, which was already strained across the country.
"So school leaders are either having to double up classes, put management staff in classes, which impacts on other parts of running the school, or rostering some years home."
The winter illnesses hit schools in waves, Miles said.
"There are different communities that are hit harder than others for a period of time, and it takes 5-10 days to get over this flu, so in that time you've got a number of children who are sick, a number of children who are about to get sick and of course you've got staff who are sick as well."
He said when illnesses were around, the principal's federation encouraged children to stay away if they were unwell, but to come back as soon as they were feeling better.
Miles said that it was up to principals to manage school policies for mask wearing and other sanitisation methods.
The 10 per cent loss in class time had a big impact, and Miles said it meant that the learning was being affected.
"So you've got to work out what the children have missed in that time that they've been away. It's about the teachers knowing their children, knowing what they have missed and looking at what they can do to catch up."
-RNZ