Contractors on Sydney's WestConnex have handed the Berejiklian Government a $1 billion compensation bill, the New South Wales Opposition has said.

The demand is mentioned in a recent report which is marked "Cabinet-in-confidence" and has been made public by Labor leader Luke Foley.

While the report speculates Roads and Maritime Services should be able to reduce the bill substantially, Mr Foley said it showed the Government had botched planning on the project.

The claims are for variations in contracts, including unforeseen planning conditions and planning approval delays.

According to the documents, Roads and Maritime Services expects to be able to negotiate the claims down from $1 billion to between $180 and $340 million.

Mr Foley said the revelations were evidence the Government's handling had made the $16.8 billion project "shambolic".

"The Premier says this is her strong suit, this is what she does well, I'd hate to see what she's weak at," he said.

He said the documents also revealed the first part of the project, the widening of the M4 from Parramatta to Homebush, was running behind time and over budget.

"You just shudder to think what the status will be of the more complex stages of WestConnex," he said.

Labor's Planning and Infrastructure spokesman Michael Daley said the Government was designing the toll road as it went.

"There's a way of doing these things that protects the public purse and this Government has just turned the process on its head," he said.

"They're like a big arrogant bull at a gate and their arrogance and their mistakes are being born by the taxpayer."

Mr Foley said the Government should have released all of the WestConnex documents itself.

"If it takes whistle blowers within Government alarmed at the cover-ups to leak these revelations to the State Opposition, then we'll happily share those … over the coming days."

Minister for WestConnex Stuart Ayres has denied Labor's claims that there is a massive cost blowout on the project.

Mr Ayres said compensation claims by contractors had been factored in to the cost of the motorway.

"Anyone who delivers large-scale infrastructure projects knows you need to budget for claims variations and compensations in your overall budget and that's exactly what the Government has done," he said.