Upgrades to the Swan Street Bridge will blow out another six months after workers discovered 70-year-old plans for the bridge were incorrect.

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.