The builders of Melbourne’s West Gate Tunnel Project say design errors caused delays and significant cost blowouts on the trouble-plagued toll road, which is already three years late and $4 billion over budget.

Construction giants CPB Contractors and John Holland are now suing the engineering firms that designed the project, in a bid to recoup losses they claim are from patching up previously undisclosed flaws on the massive project.

After being bogged down with a toxic-soil crisis, cost overruns and legal squabbles, the $10 billion highway connecting the West Gate Freeway in Yarraville to CityLink in Docklands is due to open in 2025.

The toll road, dreamt up by CityLink owner Transurban to provide an alternative to the West Gate Bridge, consists of twin four-kilometre tunnels under Yarraville, and about nine kilometres of twin elevated highways and a flyover soaring above Footscray Road and across the Maribyrnong River.

But the construction joint-venture (CPBJH) has revealed it has run into further woes building the project and pinned the blame on engineering firms Aurecon and Jacobs, which it hired for design work in 2016.

CPBJH alleges in a writ filed in the Supreme Court in December that it identified a string of design errors, including incorrect “torsional stiffness” of steel trough girders installed on four bridges that hold up the elevated motorways.

There were also “numerous errors” that resulted in a “significant increase” in steel required for a bridge near the Maribyrnong, while steel gantry designs did not comply with Victorian road specifications.

Some of these issues were discovered only after construction, and the builders say they have incurred “significant costs” amending designs and carrying out “physical rectification … to numerous affected structures”.