This vomiting illness was first reported among boarders at the Boys' High hostel, Adams House.
The school notified the Christchurch District Health Board of the cases on July 28.
It has been established the ill boarders played sport with day pupils over the weekend, accelerating the spread of the virus.
Boys' High informed parents its absence rate was at 20 percent. Usually it's about 8 percent.
Senior assistant principal Nigel Vernon said he did not know how many of the boys were sick sick with Norovirus.
The school contacted the public health team earlier this week to advise of the outbreak. The school was now keeping an illness log.
"It's a matter of monitoring our attendance rate and making sure the boys keep up following good hygiene practices," Vernon said.
This news comes a week after an outbreak at Lauriston School near Ashburton affected more than half of students, closing the premises.
Christchurch District Health Board communications team leader Mick O'Donnell said there was no established link between the two schools.
"There's always gastro around somewhere."
Norovirus can causes acute viral gastroenteritis, which is a stomach illness of sudden onset that lasts for one or more days.
More information about viral gastroenteritis can be found online.
