Tunnelling on Sydney’s M6 project, once dubbed the potentially “unbuildable motorway,” is set to resume after the state government reached an agreement with contractors, ending a two-year standstill. Work was halted after sinkholes and a reverse fault disrupted construction.
Deal Reached to Complete Twin Tunnels
The New South Wales government and the CPB Contractors-led consortium have struck a deal to finish the $3.1bn project. Under the new terms, the private consortium will absorb the costs of completing the tunnels, limiting the financial impact on taxpayers. Motorists will still have to wait, with the M6 now expected to open in 2028 at the earliest.
The project, consisting of twin 4km tunnels connecting Sydney’s south to the wider road network, was initially approved in 2019 under the former Coalition government. The original completion date of 2024 was later pushed back to 2025.
Sinkholes and Reverse Fault Caused Delays
Tunnelling was 90% complete when two large sinkholes opened in March 2024 above the tunnel and below an industrial estate in Rockdale, throwing the project into disarray. Underground works on the tunnels linking Kogarah and Arncliffe ceased altogether in June 2025 after the discovery of a “high-angle reverse fault” in the bedrock near the sinkholes.
The crisis escalated in May 2025 when a leaked email from David Jackson, director of the project’s first stage, revealed that the joint venture was pulling out. Friday’s agreement aims to settle the two-year dispute between the government and the consortium over who should pay to complete the compromised tunnels.
Government Confirms No Extra Cost to Taxpayers
The government stated that the consortium—a joint venture of CPB, Ghella, and UGL, known collectively as CGU—had agreed not to pursue contractual claims against the state for costs related to the 2024 sinkholes. With 250 metres of excavation remaining on the main tunnel, NSW Motorways chief executive Camilla Drover said CGU would complete the project under the contract terms.
“CGU will resume work on the project immediately, and we will provide an updated timeline for expected completion once we have it,” she said.
NSW roads minister Jenny Aitchison confirmed that work would resume immediately at no additional cost to the public. “Unlike the Liberals, we will not hand out blank cheques when contractors try to take NSW taxpayers for a ride,” she said on Friday.
Transport for NSW secretary Josh Murray added that CGU would be responsible for rectifying the “subsidence issue that originally paused the contract.”
Jet Grouting Method Proposed
To complete the remaining 250 metres, contractors may use “jet grouting,” an engineering method that uses high-speed fluid to erode soil, with the resulting cavity filled with grout to form a composite material. Engineer Grahame Campbell, who project-managed the M4, welcomed the resolution but said the situation was avoidable.
Campbell, author of a research paper on infrastructure bungles, said jet grouting was the only viable option. “It’s good to read that they have finally decided to sort out the contract after this protracted delay,” he said. He noted that the two-year pause highlighted a systemic failure by governments to conduct necessary preliminary geological work before signing major infrastructure contracts.
“They should have done more investigation into the geology under the road system,” Campbell said. “That’s the main reason it went over budget. Disputes are common because the government doesn’t do the preliminary work necessary to fully define the terms of the contracts.”
