The Mayor of Auckland is putting the hard word on KiwiRail over this morning's complete rail shutdown.

All Auckland trains ground to a halt this morning about 7am, and only started returning to normal this afternoon.

Phil Goff says the fault lies solely with KiwiRail, not Auckland Transport.

Acting KiwiRail acting CEO Todd Moyle says the main system and the backup signal system failed.

"We are extremely sorry for the disruption and inconvenience this has caused, we know how important the rail service is to Auckland. We wanted to make sure we are safe and fit for purpose before we ran to full operations. There was an issue with the computer system which controls everything, that seems to be the issue at the moment."

When asked if heads would roll following the drama, Moyle said it wasn't the result of human error, it was a technical fault.

"It was installed in 2011 and has operated incredibly well since then, which makes today's event very unusual. This is one of the most significant issues we have had with Kiwirail. We just need to get the trains operating again, its an equipment failure, not a individual."

Phil Goff told Mike Yardley KiwiRail's assured him they're investigating it and are planning a full upgrade of the signal system within the next 18 months.

"We need to know that this sort of thing won't happen again, and we have a system which is fit for the 21st century. KiwiRail has to run a fit-for-purpose operation."

LISTEN ABOVE AS PHIL GOFF AND TODD MOYLE SPEAK TO MIKE YARDLEY