Auckland's trains have resumed operating after a "signalling issue" paralysed the city's networks this morning.

Auckland Transport said the KiwiRail signal issue had been resolved and services were slowly returning to normal.

Trains were at a standstill for more than 30 minutes today - plunging the city's morning commute into chaos.

However, the Onehunga line remains suspended.

A fuming Auckland Mayor Phil Goff said a second KiwiRail signal failure on Auckland's network was "not good enough".

"I have asked KiwiRail for more information and sought assurance from its chair Greg Miller that infrastructure is fit for purpose," he said on Twitter.

The network failures threatened to tarnish the image of Auckland's public transport.

"We are promoting public transport, we are getting a big lift in the use of public transport - and therefore we need to be able to rely on it," Goff told the Herald.

A second KiwiRail signal failure on Auckland’s network is not good enough. I have asked KiwiRail for more information and sought assurance from its chair Greg Miller that infrastructure is fit for purpose.

— Phil Goff (@phil_goff) June 18, 2019

"When you have two problems both relating to signals on KiwiRail infrastructure... we need an explanation and assurance this isn't going to be an ongoing problem."

The problem appeared to be linked to issues supplying power to KiwiRail's signalling equipment, Goff said.

"That simply was not good enough."

However, Goff was pleased KiwiRail was at least able to get the system back up and running quickly this morning.

KiwiRail's Miller had also informed Goff about both recent train failures immediately after they happened, he said.

"People at this hour of the day are travelling to work, they need to be at work on time or they are travelling to appointments and what we want to be able to do is guarantee to our commuters that this service is reliable to the maximum extent and that reliability is challenged by having two events relating to signals within the space of a week."

The scene at Manukau railway station as commuters are forced to catch buses.

Goff expected KiwiRail to get to the bottom of what was going on, resolve it - and give Aucklanders the assurance it wouldn't be an ongoing problem.

"I'm very pleased they got the service running up quickly but that was twice within a week they've had a signals failure on KiwiRail equipment ... and that simply should not be happening."

Auckland Transport confirmed just after 8am that trains across the entire city network were not running due to a KiwiRail "signalling issue".

"It is unknown at this stage when services will resume," AT said in a tweet.

"Buses are accepting purchased train tickets and AT Hop cards."

One commuter told the Herald an announcement on her train's intercom said there was an Auckland-wide rail system failure.

She was left stuck in between two stations near New Lynn, with her packed train pulled up to a standstill.

The train sat still for about 15 minutes before rolling into New Lynn station as grumbling passengers phoned in to their workplaces to say they would be late or have to miss meetings.

At New Lynn station, most people stayed on board briefly, hoping the train would get moving again, until an intercom announcement told them it wasn't going anywhere.

Fortunately a number of regularly scheduled buses were nearby as the train passengers piled on.

She said she was now running late for work - but at least she was moving again.

Another commuter told the Herald she was stranded at Manurewa after hearing about the train outage.

The message board at Britomart. Photo / Michael Craig

Herald journalist Cherie Howie was on her way to Manurewa Station when she heard there was an outage and asked commuters milling around what had happened.

She has now instead hopped on a bus to Manukau in the hope the trains get started again and she can catch a train from there.

It "amazingly" wasn't too crowded on the bus just yet, she said.

"It does make me think that driving and parking or taking an Uber or a bus is getting to be more reliable, despite it adding to Auckland's congestion," she said.

Delays were already expected this morning on the Southern line after Auckland Transport announced yesterday that there would be some service cancellations and delays this morning.

Several delays and cancellations have occurred on AT's train services over the past few weeks, mainly on the Southern Line.

This morning's train failure comes amid news that Auckland's traffic woes are the second-worst in Australasia, with only Sydney faring worse.