Fire and Emergency New Zealand (Fenz) shift manager Riwai Grace said they received multiple calls in relation to a fire at South New Brighton School about 6.40pm.
"Crews arrived to find three classrooms and one part of an office block well ablaze."
A second alarm was used to call more firefighters to the school and at the fire's peak seven fire trucks were used to battle the flames.
The fire was under control by 7.30pm, Grace said.
The school posted on Facebook it would be closed on Monday.
"A huge thanks to staff and community for offers of support," the post continued.
"If anyone has information please contact us privately or the police on 105."
Steph Hickman has two children at the school and was watching the fire from police cordons set up around the school.
"When we came over Bridge Street you could just see all the black smoke," she said.
"There's a lot of families have come down ... a lot of families live in the area."
A scene guard was in place overnight and police and Fenz investigators were on the scene this morning.
- additional reporting RNZ
