Buckland School, near Pukekohe, said the lockdown was due to an “adult medical emergency”.
The school says after-school pick-ups will go ahead at the normal times, but students will be escorted out to parents.
A spokesman for police said an adult had died on the school grounds.
“There are no suspicious circumstances, and police will undertake our usual inquiries,” the spokesman said.
A notice on the school’s website says a “long-time visitor” to the school had died.
“It is with sadness that we inform you. Our thoughts are with the family,” the notice said.
“The children are now outside having a play and will be placed back into classrooms at the direction of police.
“Pick-up will be at the normal time. Please refrain from entering the school grounds. Students will be brought to you.
“The students have been spoken to about the incident and support will be available should they need it.”
An earlier notice said all children were safe and told parents not to phone or come to the school.
“Your assistance and co-operation are appreciated at this time,” it said.
The police spokesman said: “Other agencies are being notified and support will be put in place”.
