The mayor of Auckland is demanding answers, after the city's trains were shut down in the middle of rush hour due to a major power outage in Wellington.
The outage at National Train Control hit around 4pm yesterday afternoon after backup systems failed.
It lasted for about an hour but caused disruption for much longer as commuters attempted to catch trains home.
Once signals were restored, commuters faced lengthy delays as trains got moving again.
Mayor Len Brown said the problem had been disappointing at a time when hundreds of millions of dollars were being spent on upgrading Auckland's rail network.
"We've a got fully integrated electric rail system coming into position mid next year, and we do not want any of these types of control issues impacting on the delivery of super modern service."
Len Brown said he expected a report into what happened on his desk this morning.
Meanwhile KiwiRail has been investigating what caused the power outage.
Chief executive Jim Quinn told Newstalk ZB this morning that the incident had been unusual and had not happened before.
He said he was expecting a report on what caused the power outage and why the backups did not function as they should have.
"We'll investigate this as quickly as we can and run to ground what's both caused the issue and secondly what's meant that the backups didn't kick in," he said.
Mr Quinn said there was no use speculating on what happened until all the facts were clear.
"But don't for a minute think we didn't have backup - we do. There is a clearly an odd set of events that have caused this.
"What we don't know is why, and once we've got that sussed of course the next thing is to make sure we implement a remedy that fixes that in the long term."
He defended the centralisation of train control in Wellington, saying that was not the cause.
"We centralised train control a long time ago but the purpose of doing that is to centralise the investment you've got to make, to make a 24 hour business work," he said.
"And obviously in the debate that we have right now, that hasn't shined as well as it should. But I should say remoteness actually has nothing to do with this. Whatever has caused this could have occurred whether it was in Wellington or Auckland."
Mr Quinn said teams had worked through the night to make sure the system was working as it should and had some backup capacity.