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Principal Vivienne Butcher said the school immediately took action following the incident and adjusted the doors to ensure they closed more softly.
The girl was a former pupil who was visiting a teacher with friends. She was getting ready to start at Lincoln High School. The incident happened on January 29.
Selwyn Times understands the girl underwent plastic surgery in a bid to restore the finger.
The school is currently monitoring its automatic door closers to determine if they need replacing.
Mrs Butcher said there is a high likelihood if the doors cannot be adjusted to prevent future accidents, the devices will be replaced to ensure they close as softly as possible.
"We don't want to leave anything that is going to cause serious injury or harm."
Mrs Butcher said the building where the incident happened was commissioned in 2010 and the closing devices are the highest specification available.
"There have not been any similar incidents," she said.