Sydney’s ‘unbuildable motorway’ to be completed after two-year delay caused by sinkholes

Tunnelling on Sydney’s $3.1bn M6 project. dubbed the potentially “ unbuildable motorway ”, will resume after the state government struck a deal with contractors, ending a two-year stalemate during which works were halted.

Under the terms of the new agreement. the private consortium will absorb the costs of finishing the tunnels, limiting the financial hit to taxpayers.

Motorists will still need to wait, however, with the M6 now scheduled to open in 2028 at the earliest.

The project. which consists of twin 4km tunnels connecting Sydney’s south to the wider road network, was initially approved in 2019 under the former Coalition government with an original completion date of 2024. That was later pushed back to 2025.

Tunnelling was 90% complete when two large sinkholes opened in March 2024 above the tunnel. 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. The consortium charged with completing the M6 said it would down tools after the discovery of a “high-angle reverse fault” – where a layer of rock is forced upwards. over another layer – in the bedrock close to the sinkholes.

The crisis was ventilated in May 2025. when the Sydney Morning Herald published a leaked email to staff from David Jackson, the director of the project’s first stage, stating that the joint venture was pulling out.

Friday’s agreement looks to settle the two-year dispute between the New South Wales government. the CPB Contractors-led consortium over who should pay to complete the compromised tunnels.

The government said the consortium – a joint venture of CPB, Ghella. 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. the NSW Motorways chief executive, Camilla Drover, said CGU would complete the project as per the terms of the contract.

“CGU will resume work on the project immediately,. we will provide an updated timeline for expected completion once we have it,” she said.

The NSW roads minister, Jenny Aitchison, said work would resume immediately and at no additional cost to the public.

“Our priority has always been to complete this project under the existing contract,” she said on Friday. “Unlike the Liberals, we will not hand out blank cheques when contractors try to take NSW taxpayers for a ride.”

The breakthrough follows a revelation that the government reportedly explored abandoning the project during the standoff.

Josh Murray. the Transport for NSW secretary, said CGU would be responsible for rectifying the “subsidence issue that originally paused the contract”.

“While there has been a regrettable delay, the contract for the M6 Stage 1 has clear dispute resolution processes. these will be used,” he said in a statement.

CPB was contacted for comment. CPB’s executive project director, Terry Sleiman, told the SMH the agreement provided the “opportunity to continue tunnelling works”.

To complete the remaining 250 metres. contractors could use an engineering method known as “ jet grouting ”, which employs high-speed fluid to erode soil, with the resulting cavity filled with grout to form a composite material.

Grahame Campbell, an engineer who project-managed the M4, delivering it in half the forecast time. budget, welcomed Friday’s resolution but said the situation was avoidable.

Campbell, who authored a research paper for the Centre for Independent Studies about “bungles” that drive up project timelines. costs across Australia, 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. “[Jet grouting] is used to solidify the rock so it can be drilled and tunnelled.”

Campbell said 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,” he said on Friday.

“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.”