KiwiRail has admitted it wasn't fully accurate in communicating the reasons behind Wellington's rail woes.
Train services in Wellington were drastically cut back on Monday after the breakdown of a specialist rail track evaluation car.
It prompted speed restrictions of 70km/h, with just half as many services as usual during peak times.
Transport Minister Michael Wood announced a rapid review into KiwiRail on Monday after a meeting with bosses.
KiwiRail put out a press release last week saying the reason for the disruptions was because of a mechanical issue.
But the Transport Minister revealed to AM earlier on Tuesday the main reason for the disruption was because of staffing and rostering issues.
"My understanding of the advice I have is that actually what has mainly driven the fact that this job wasn't done at the right time in Wellington was a scheduling and rostering issue," Wood told AM.
"So effectively there's been a dropping of the ball within the organisation around that and that's why the review is important because that speaks to the systems not working appropriately and not being sufficiently focussed."
When pressed by AM co-host Ryan Bridge if KiwiRail was 100 percent accurate in the press release they sent out last week, CEO Peter Reidy admitted they weren't "fully" accurate.
Reidy said the reason why rostering and staffing issues weren't mentioned in the press release was because they didn't see it as the "primary cause" at the time.
"When we sent the press release out, the machine was down and that was the primary reason. We spoke to Metlink and said we can't get the machine to Wellington," Reidy told AM co-host Ryan Bridge.
A combination of staffing, rostering and mechanical issues forced KiwiRail to reduce services and enforce speed restrictions.
"It's a combination of two things. Firstly, it is a work scheduling and process compliance gap, an issue that we're investigating and secondly, we had a machinery failure that stopped us from doing the work and the window that we hit and that's why we had to put speed restrictions on the line," he said.
Reidy said the specialist rail track evaluation car only operators in Wellington three weeks every year and KiwiRail only has one of these.
He told AM when it was Wellington's window for track inspection, the specialist machine was broken.
"You can only do it at night time, so you've got windows to operate. If there are mechanical failures, you've got to move the machine. Sometimes there will be inspection issues in other parts of the network, so you've got to prioritise the two," he said.
"What happened here was there were a couple of windows we missed. We had some staffing issues, the mechanical failure happened a couple of times and we missed the window.
"You get a couple of shots at it, but we let ourselves down, dropped the ball, and we didn't give ourselves enough window to get back in."
The specialist rail track evaluation car requires people with specialised skills to operate the machine and when it was Wellington's turn, they had rostering issues.
"We have a big rostering program that will have people to operate this machine, bear in mind, it's very specialist equipment, so we don't have teams of people on standby," he said.
"We have a very specialist team of people to do it, so we have rostering for that and secondly, if we have mechanical failures at times, we've got to stand those people down and get it fixed.
When pressed by Bridge if someone should be fired over this botch-up, Reidy didn't believe so and was proud of his team.
"If you think about it, it's the first time we've had a failure in 41 years. We carry 26 million people a year and 26 percent of New Zealand's exports," Reidy told AM.
"Every day I'm really proud of my people and what they do. There's an absolute drop of the ball here and we're investigating that."
Reidy said work has started on repairing the lines and they hope to have services back to normal by the end of the week.
"It started work last night. It's in Wellington now, it's going to complete up to Waikanae and back to Wellington tonight. We'll have the lines finished by Thursday and the rest of the network by the week."
Watch the full interview with Peter Reidy in the video above.
