The city's mayor says he will be asking Auckland Transport for an explanation about how the city's rail network was crippled for most of the day, and how it could be prevented from happening again.

Photo: RNZ / Laura Tupou

All Auckland trains were halted this morning after a signals system failed just before 7am. The backup signals system also failed.

Thousands of commuters were left stranded trying to get to work, and others were left stuck in immobile trains, waiting to get out.

Buses were brought in on key routes to take extra passengers during the day.

Trains were running every 30 minutes by lunchtime and Auckland Transport said that was down to every 20 minutes for the afternoon peak.

Trains are now operating every 20 minutes on the Southern, Eastern and Western Lines. The Onehunga Line is running as normal: https://t.co/vaY71gsMuC — Auckland Transport (@AklTransport) January 25, 2019

Auckland Transport said the network would be back to normal service tomorrow.

KiwiRail operates the rail signals, but a spokesperson said it does not know what caused the fault, or when everything will be back up and running, but a team is investigating.

Auckland Mayor Phil Goff said he would be meeting with the city's transport agency Auckland Transport to discuss the fault, and how similar problems could be prevented in future.

People waiting at the Ōrākei train station were frustrated by a lack of communication, commuter Cathy Clark said.

"There was no way that I found out that the trains weren't running - it wasn't from a member of Auckland Transport at the train station - it was from a member of the public who told me the trains weren't running. So that was a little frustrating," she said.

Monessa Thompson was late to work, but made it eventually.

"I was there from 7 o'clock waiting for the train, but they said the lines were down, so we had to catch a bus - we waited 20 minutes, and then it took us an hour from Middlemore to [Britomart]."

The trip would normally take about 30 minutes.

Photo: RNZ / Liu Chen

KiwiRail said it had never experienced a fault like this before.

The system had been operating since 2011, and both the main signals system and the back-up have been in regular use, KiwiRail said.

It said it was looking into the cause of the failures of both systems.

Auckland Transport said passengers would be given automatic refunds for customers who have a registered AT HOP card and whose travel was disrupted.

Other affected customers would need to go to a customer service centre for a refund.