The builders of Melbourne's trouble-plagued West Gate Tunnel are suing the mega road project's engineers over claims design errors smashed its bottom line.
In a writ filed to the Victorian Supreme Court in December, CPB Contractors and John Holland allege engineering firms Aurecon and Jacobs' design for the project contained errors and described its services as deficient.
The joint venture identified a host of alleged design errors across six bridges, including a miscalculation on the stiffness of steel girders and deficiencies that led to a significant rise in steel tonnage.
Each of the alleged errors caused the builders to shoulder "significant costs" to correct designs and meant it had to physically rectify "numerous affected structures", delaying and disrupting the performance of works.
"The plaintiffs are continuing to investigate further design errors as the project progresses," the writ reads.
Aurecon and Jacobs were hired in 2016 to carry out the design work, with the companies relying on it to prepare a bid for the tunnel project.
The joint venture claimed the $6.7 billion fixed-price contract it entered into with project manager Transurban and the Victorian government was "inadequate" due to the alleged design errors.
CPB Contractors and John Holland are seeking damages and compensation from the firms, but did not specify an exact amount.
Aurecon declined to comment. Jacobs has also been contacted for a response.
A hearing date for the matter is yet to be scheduled.
Deputy Premier Ben Carroll told reporters on Monday the tunnel is still expected to open in late 2025 and the dispute was one between the builders and their suppliers.
A second crossing to and from the city's west over the Yarra River, the tunnel was initially scheduled to be completed in 2022 before being plagued by delays and cost blowouts.
The discovery of toxic soil stopped digging and sparked community anger over its dumping northwest of Melbourne.
A long-running dispute over the bill for the extra costs was settled in late 2021.
Under the terms of the deal, Transurban agreed to pay an extra $2.22 billion and the builders waived $1 billion in revenue.
The state government chipped in another $1.9 billion, taking the tunnel's total cost to $10.2 billion for Victorian taxpayers.
