Two other students at the Pitt Town Public School and a teacher were also trapped under the branch, the Sydney Morning Herald reported. The children suffered minor injuries, while the teacher was more seriously hurt.
Bridget Wright, a fourth-year student, died at Westmead Hospital.
The newspaper said the eucalyptus was about 50 years old and very high.
Eucalyptus, also known as gum trees, are indigenous to Australia. Some species have a reputation for dropping branches, and Parks Victoria recommends against camping under some species. The wood is also heavy because it is dense and full of resin, making falling branches more dangerous.