KiwiRail's chief operations officer, Todd Moyle, acknowledged that the investigation was ongoing and would take some time.
The organisation is working closely with TAIC and said the incident could have been caused by a "whole lot of factors''.
"Passenger train derailments in New Zealand are a rare occurrence.
"The last one was in 2013, so we will be looking to learn from this event once a thorough investigation has been completed and the cause has been ascertained.''
Moyle said although the derailment was minor, it happened at a complex spot and as a result, caused real disruption to passengers.
A total of 33 people were on board the six-carriage train when three carriages went off the tracks as the train was leaving the station.
Those on board said it scared passengers, describing the accident as a sudden shake that felt like a small earthquake.
No one was injured in the incident.