A legal fight is brewing over the problem-plagued M6 motorway tunnel, with the contractor working on the $3.1 billion project threatening to walk away.

Now, there are claims that a major issue with the ground has made the project more complex and expensive.

CPB Contractors says the sinkhole was caused by “unique adverse ground conditions caused by a complex faulting zone … never seen before in the Sydney Basin”.

CPB says it could not have been anticipated and was only discovered when tunnelling began in an area near the sinkhole.

Plans to remediate the issue caused by the contractor are still yet to be presented.

“Send the lawyers home and bring back the engineers, we need to get a solution to this,” NSW Premier Chris Minns said. 

The $3.1 billion contract signed by the previous government is for the design and construction of the tunnel.

If CPB refuses to complete it, Transport for NSW could take it to court.

“We would be very disappointed if this $3.1 billion contract ends up in court or with one of the country’s largest construction companies walking away,” NSW Secretary of Transport Josh Taylor said.

“We’ve got a lot of big projects taking place across NSW, and I need to maintain the integrity of these contracts because it’s not my money, it’s the taxpayers of NSW,” Minns said.

A legal fight would likely cost taxpayers millions.

If the government is forced to find another contractor, that would be even more expensive.

The delays are bad news for Sydney drivers and frustrated locals, who have endured five years of construction. 

The project is 90 per cent complete, with transport bosses confident they can find a fix.

They are due to meet tomorrow.

“There’s more homework to be done, we’d like to review other design components here from the contractor, that is their responsibility to bring forward new design solutions,” Murray said.