"We've been advised by Health and the Ministry of Education that he has potentially been infectious [on] Monday and Tuesday while he was at school," Day said.
"At this stage we're still working though [the contact-tracing] process but we expect there's between 20 and 30 students and possibly up to 10 staff.
"We are closed so we can manage the contact-tracing process ... and collect all the data for public health."
Day said all staff at the school were vaccinated, including the staff member who tested positive.
"He was fully vaccinated and and he received his second dose I think just over a week and a half ago."
The second Nelson school, Broadgreen Intermediate, has told parents it had a positive case who was infectious from last Thursday.
There are number of close contacts who have been told to get tested immediately and isolate at home.
Director-General of Health Ashley Bloomfield told Morning Report the latest group of cases in Nelson are separate from the first case in Nelson. They relate to a person who had several workplace and flatmate contacts.
"Through the contact tracing and follow-up testing ... [we] identified a couple of staff members at different schools who were [tested and received positive results], recalling that all staff at schools are now required to be vaccinated."
The staff were not symptomatic, he said, and the schools had followed the guidelines.
The last day employees in the education sector had to get at least one dose was 15 November, and they are required to be fully vaccinated by the end of this year.
"One of the reasons the mandate was put in place was to help protect children in schools [while they cannot get vaccinations]," he said.
"The vast majority of our cases that are under the age of 12 in the outbreak to date right across the country, but obviously largely in Auckland, had been infected in the home by other household contacts, not in school or early childhood education setting."
Health measures would be used rather than any lockdown being needed in Nelson, Bloomfield said.
Nelson's high vaccination rate and other measures like contact tracing and isolation rules would provide a high level of protection for the region, he said.
"We'll use our contact tracing and testing and isolation as we have in other places."
- RNZ
