Rangiora High School has more than 1600 students and 120 staff.
Principal Bruce Kearney said they could have 300-400 students away on a day, and up to 30 staff were off sick.
"It seems to be a merry-go-round with the sickness. Just when you're coming right you get sick again and it seems to take a little bit longer," he said.
He did not rule out shutting the school if there were not enough staff, and said he had not felt the pressure to do that yet this winter.
"We have 120 teachers, I'd say if we're getting over 35 (off sick) then that becomes a reality, simply because we can't get relief teachers. There are not enough relief teachers to cover so many classes," Kearney said.
Other schools were also feeling the impact of winter illness, he said.
"Most definitely heard that this is probably a little bit worse than normal, but not massively worse. I want to put that into context, things are a little bit worse but a little bit worse can make a big difference."
He urged people to keep sick children at home to stop illness from spreading.
But that could be a challenge when the school had been messaging its community constantly about increasing attendance to meet the government's goal, he said.
Associate Education Minister David Seymour has previously said the government aimed to ensure 80 percent of students were present more than 90 percent of the term by 2030.
"So we've been talking about coming to school and then of course we hit a winter like this, and attendance just massively falls away and it just shows how difficult the whole thing is," Kearney said.
