The tree's exact age wasn't known but the school celebrated 95 years this week so she believed it could have been planted then.
Trees on the school grounds were checked routinely, Ms Gunn said.
An Auckland Council spokeswoman last night said a council arborist who examined the tree found it was affected by white fungi. A consent was granted in 2010 to prune it.
Ms Gunn said some of the school's property staff were planting close by when the tree came down. "It happened very suddenly. They heard a crack and then it fell, but there wasn't any warning."