The train derailment at Auckland's Britomart station last month has led to a mammoth repair bill.

An investigation into the cause of the incident on the morning of May 9 continues and is being led by the Transport Accident Investigation Commission.

The derailment proved to be a nightmare for passengers on the train network that day and for regular users as the station had to be closed for several days after the incident.

Now KiwiRail has revealed it has had to fork out $44,000 to fix the issue.

The cost is made up of $15,000 which was spent on materials to repair the damage and $29,000 in labour costs.

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.