Human error has been blamed for a truck rollover which caused traffic chaos and hours-long delays on a major city freeway.

A metal plate — left covering a bridge expansion joint when routine maintenance work was paused for the day — came loose and created an obstacle which several vehicles, including the truck, then hit, the Department of Transport and Planning said.

The truck rollover early on Wednesday morning closed all city-bound lanes of the Princes Fwy at Werribee in Melbourne’s outer southwest during peak hour.

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With delays of more than four hours, the DTP was warning warned drivers to take another route — or public transport — into the city.

DTP executive director of road maintenance Michael Bailey said the damage was caused by human error, with the loose plate believed to be directly linked to the truck rollover.

He said during ongoing routine maintenance, steel plates are typically used to cover areas being worked on so the road can be re-opened to traffic.

“In the events of maintaining the expansion joint on the bridge, a steel plate has come loose and come off the road,” he said.

“Vehicles have subsequently hit that, and that has caused significant delay to road users.”

The plate was installed about 3am, just before work crews left the site, while the truck rollover was reported at 4am.

“One (steel plate) has come loose, and we believe it is through human error, and subsequently that has caused the damage,” Bailey said.

“We appreciate it got up to about four hours in delays and we apologise unreservedly.”

Commuters were urged to detour via Bacchus Marsh and the Western Fwy or take a V/Line train to avoid congestion.

All city-bound lanes were eventually reopened although delays remained for some time.

A reduced speed limit of 60km/h is in place until crews can return on Wednesday night to complete repairs to the road surface.

Drivers whose cars may have been damaged should call the DTP on 13 11 70 regarding possible compensation.

“We will work directly with those impacted and go through the process ... we will work directly with people,” Bailey said.