Roads Minister Luke Donnellan said he felt "like a fool" after discovering the $30 million project, due to be finished last month, would need to be redesigned and would not be finished until January 2018.

The works to add another eastbound lane and widen footpaths on the Swan Street Bridge have created massive congestion in the Richmond area since construction began in March, with travel times ballooning by almost 20 minutes in peak times.

Mr Donnellan said he was embarrassed by the setback but said surveys done at the start of the project did not reveal any issues.

"The Swan Street Bridge is 70 years old, and as we've opened up the bridge we've found the historic, technical drawings that guided the final design are inaccurate," he said.

The plans - which the government has refused to release for security reasons - were drawn by hand in the lead up to the 1956 Melbourne Olympics and misrepresent the location of steel within the bridge structure.

It's believed an existing gas line was also absent from the blueprint. This means widened parts of the bridge have had to be redesigned.

Swan Street Bridge provides access to some of Melbourne's most popular venues, including the MCG.

Mr Donnellan said lane closures would remain in place until October. It's hoped the project will be finished in time for January's Australian Open, but it may take longer.

"Our first priority is to get the bridge back to four lanes - we're on track to achieve that in October," he said.

VicRoads is working with contractor Seymour Whyte to deliver the upgrade as fast as possible. Seymour Whyte also has the $56 million contract to widen the Chandler Highway Bridge at Alphington.

Mr Donnellan refused to comment on how much the final cost of the project would be.

"There will be a cost overrun, there is no doubt about that, but it certainly won't be in the tens of millions," he told radio station 3AW.

"It's terribly embarrassing for me as the minister. I feel like a fool but let's be blunt, we need to get that extra capacity on the bridge. It's a pinch point."

"I'd rather have a delay and get the project done than not get the project done at all."

The four-lane bridge has been reduced to three lanes following the closure of one citybound lane, sparking heavy delays.

More than 30,000 cars, 1000 pedestrians and 1000 cyclists use the bridge a day.

Shadow minister for roads and infrastructure Ryan Smith accused Premier Daniel Andrews of failing to deliver on a $60 million election promise to fix gridlock on nearby Hoddle Street and Punt Road.

"Three years later traffic is getting worse, with this latest stuff-up just adding to drivers' woes.

"Labor's poor planning continues to feed Melbourne's congestion crisis, with the only thing moving slower than the traffic being Daniel Andrews' plan to fix it."