Students have been kept in isolation at Wanganui Collegiate School after a nasty gastro bug hit the premises.
Whanganui Public Health medical officer Patrick O'Connor said about 100 students had been unwell since Sunday, mainly with vomiting and some diarrhoea.
"We do not know the exact cause, but a virusis likely. Lab tests may give us answers by early next week," Mr O'Connor said.
He said viral gastroenteritis circulates in the community, especially in the winter months.
"It spreads most rapidly in settings such as rest homes and boarding schools where people live in close proximity," Mr O'Connor said.
Isolation areas have been set up in the boarding houses to minimise transmission and staff are continuing to observe over the next 24 hours.
Mr O'Connor has advised the school of a number of measures including isolation of cases, hand-washing, laundry, cleaning, and provision of food.
"The numbers of new cases have decreased each day, and we are aware of three new cases today," Mr O'Connor said.
The school said they were happy to report that the well-being of the majority of the students has returned to normal and only six students were still in an incubation period, on the advice of the WDHB Public Health Services.