The second stage of the WestConnex toll road project is at risk of running a year late and more than $700 million over budget, according to leaked confidential documents.

Details of the expected cost blow out and delays to construction are detailed in legal letters revealing a contract dispute between the contractors and the Sydney Motorway Corporation, the government entity charged with delivering Australia’s largest toll road project.

The CPB Dragados Samsung joint venture, which is building the new M5 from Beverly Hills to St Peters, claims design and construction changes to the project entitle it to $79.7 million in “change costs” and more than $625 million in “contractor delays costs”, according to the letters seen by the Herald.

An artist's impression of the St Peters Interchange which will form part of the WestConnex M5 project.

A letter dated May 10, 2017, also reveals that “unforseen planning approval requirements” for the second stage of WestConnex would delay the opening of the new M5 by 400 days to January 2021.