Recent bad weather is behind the overhead line problem that caused Auckland's widespread train disruption, KiwiRail says.
Trains in Auckland are running again after a rush hour of chaos for commuters on Friday morning when services across much of the network were halted.
Southern and eastern line services between Ōtāhuhu and Britomart were cancelled, along with trains on the western and Onehunga lines.
The swathe of cancellations was caused by an power outage after KiwiRail discovered contaminants had caused arcing on the overhead lines in the Newmarket area.
KiwiRail chief operations officer Siva Sivapakkiam said the problem was discovered at 4am, and the decision to shut off the power to investigate was made.
"Apologies to the people of Auckland for the disruption [that was] caused in the morning today.
"The arcing was caused due to, pretty much, the bad weather we have had and there's been contaminants on the insulator."
Sivapakkiam said the bad weather has caused contaminants to fly into the overhead insulator and the humidity had not helped.
He said full inspections were done every six months and a full clean carried out every year.
KiwiRail was still within its maintenance schedule timeline - but admitted it may need to rethink it given the incident.
"We've never encountered this before," he said.
Sivapakkiam assured commuters that the issue had been resolved and there would be no issues for the afternoon and evening commute.
Auckland mayor Wayne Brown said he was disappointed with the cancellations.
Auckland Transport will meet with KiwiRail executives this afternoon over the issue.
Brown said the fault was the result of years of under-investment, and something that was finally being addressed.
AT earlier said it was "hugely disappointing and frustrating" that it was not able to operate services as usual.
It would seek more detailed answers from KiwiRail about how to prevent a recurrence, AT's statement said.
Cancellations 'devastating' - National
Transport Minister Michael Wood acknowledged the cancellation of Auckland trains was frustrating for commuters. He said KiwiRail had identified the problem and was working quickly to resolve it.
The widespread service stoppage in Auckland followed turmoil on the Wellington network, with train services were drastically cut back due to a breakdown in a specialist track inspection machine.
Wood on Monday announced an urgent review of of KiwiRail, saying the disruption in the capital was "not good enough and follows other similar instances of service disruptions to commuters on KiwiRail's network".
National Party transport spokesperson Simeon Brown said Aucklanders wanted to see a more reliable system.
"This is devastating for commuters in Auckland, following on from what's happened in Wellington," he told Morning Report.
"This government has put $8 billion into KiwiRail over the last few years and the reality is we want to set better outcomes from or that expenditure - making sure it's more reliable."
10,000 passengers affected
About 10,000 people were potentially affected by the disruption, AT group manager metro services Darek Koper said.
AT was told about 5:20am that trains were not able to run, which did not give it enough time to source buses and ge them to the right locations quickly enough for the morning peak.
While the fault was potentially a question of infrastructure maintenance, the arcing on the wires could have been caused by something as minor as someone throwing a wire or another object across two wires, he told Nine to Noon.
"Until we know exactly what happened we don't want to speculate too much, because it could be something not directly under the control of KiwiRail or it could be something that is related to the way they maintain the network."