Contractors on Sydney's WestConnex have reportedly handed the Berejiklian Government a $1 billion compensation bill.

The New South Wales Labor opposition says it has obtained a recent report marked ‘Cabinet-in-confidence’ that leader Luke Foley says shows the Government botched planning on the project.

But the report allegedly speculates that Roads and Maritime Services should be able to reduce the bill substantially.

The claims relate to variations in contracts, including the costs of unforeseen planning conditions and planning approval delays.

The documents claim that Roads and Maritime Services could negotiate the claims down from $1 billion to between $180 and $340 million, but Mr Foley insists the revelations reveal the Government's “shambolic” of the $16.8 billion project.

“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 claimed the first part of the project, the widening of the M4 from Parramatta to Homebush, was running over its budget and deadline.

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

Labor planning and infrastructure spokesperson Michael Daley said the Government was designing on the run.

“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 be more transparent.

“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,” he said.

Minister for WestConnex Stuart Ayres denied the claims of a massive cost blowout.

Mr Ayres said compensation claims were factored in to the cost.

“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.