The girl's mother got off at Pukerua Bay station with her two younger children on Saturday afternoon, but her eldest child was left on the train, a spokesperson said.
A passenger, who saw the girl was distressed, got off with her at the next stop and took her in a taxi back to her mother.
Tranz Metro says the normal procedure at stops is for the train manager to leave the train and check all passengers have disembarked and the doors are clear before shutting them.
It says it is looking into how long the doors stayed open and whether a train manager went to check the platform was clear.
