The cost of connecting the WestConnex motorway to Sydney's existing road network will amount to nearly $1.8 billion, a leaked government document shows.

Labor has seized on the document marked "Cabinet-in-confidence" by Transport for NSW as evidence that the real cost of WestConnex is nearly $20 billion – almost double the original price of the toll road project proposed by Infrastructure NSW in 2012.

The document leaked to the state opposition lists among the "WestConnex integration works" the $754 million cost of linking a widened section of the M4 motorway to nearby thoroughfares such as Parramatta Road, and $423 million for connecting an interchange at St Peters in Sydney's inner west to local roads.

Labor deputy leader Michael Daley said taxpayers had a right to know just how much the final cost of WestConnex would amount to because they would be footing the bill for years through tolls.