Upgrades to the congested Swan Street Bridge in Melbourne have been delayed four months after it was discovered that plans for the 70-year-old bridge were inaccurate.

Initially, the state government’s Swan Street Bridge project was due to finish last month. Plans for the project included an added lane for drivers heading in an easterly direction, and improved pedestrian and cyclist access.

However, the expected completion date has been pushed back after it was found that plans for the project had misplaced the bridge’s steel structures. These delays could potentially wreak havoc on next year’s Australian Open traffic. Even in normal conditions, it’s estimated that 30,000 vehicles use the commuter hotspot each day.

Speaking to radio station 3AW, road minister Luke Donnellan said he felt “like a fool” announcing delays as a result of bungled plans.

“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.

“We had to redesign [the Swan Street Bridge project] on the run. It is disappointing, no doubt about that.

“I feel like a fool as a minister to actually make pronouncements saying it will be done in July. But here we are, in August.”

The historic plans Donnellan refers to were drawn by hand in preparation for the 1956 Melbourne Olympics. As a result of their inaccuracy, modern-day plans detailing the bridge’s $30-million redevelopment have had to be redesigned. It is now estimated that it won’t be complete until January 2018.

Although Donnellan has said that this unexpected development would result in a cost overrun for the project, he has so far refused to disclose how much it is now estimated to cost.

“There will be a cost overrun, no doubt about that, but it certainly won’t be in the tens of millions,” he told 3AW.

VicRoads is now working with contractor Seymour Whyte to deliver the project as fast as possible, with the first priority being to get the bridge back up from three lanes to four.