Traffic on the freeway was thrown into chaos after the power lines that run beneath the Moreland Road bridge came down about 11am on Thursday.

VicRoads had closed a large section of the freeway for more than two hours.

At one stage, traffic was banked back to the Western Ring Road.

Chris Miller from VicRoads' Traffic Management Unit said his team had worked hard to keep traffic flowing.

"Delays are building fairly quickly," he said, shortly after the power lines came down.

"High-voltage lines means public safety is something we have to be very careful with."

Drivers heading towards the airport were told to exit at Mount Alexander Road at Flemington, head through Moonee Ponds and Essendon and re-enter the freeway at the Bulla Road on-ramp.

City-bound drivers had been advised to exit at Bulla Road and head towards Essendon and Moonee Ponds and re-enter the Freeway at the Racecourse Road on-ramp.

It's not yet clear what caused the power lines to come down.

More to come.