The WestConnex motorway has attracted more than $1 billion in legal claims from companies hired to build the toll road, an internal document obtained by the opposition says.

The document says Roads and Maritime Services has received from the Sydney Motorway Corporation $1.05 billion in legal claims from contractors, arising from matters such as contaminated land, unforseen planning conditions and delays in obtaining planning approval.

But the government says there will be no blow-out to the $16.8 billion cost of the WestConnex motorway, nor will it be delivered later than its completion date of 2023.

The document, marked cabinet in confidence, was used by Labor leader Luke Foley to criticise the government's handling of the mega toll-road.