There's a $20 billion black hole in the federal opposition's plan for two rail projects in Sydney's west, the Turnbull government says.

Labor leader Bill Shorten announced plans to spend $6 billion on two projects in the city if they are delivered in to government at the next election.

In a speech to the party faithful at its state conference on Sunday Mr Shorten committed $3 billion to the Sydney Metro West project, linking the city to Parramatta, which will double capacity between the CBD and Parramatta.

Another $3 billion will go towards the Western Sydney Rail project, which will connect all of the city to the new airport planned for Sydney’s west.

But Urban Infrastructure Minister Paul Fletcher said the federal government had scoped the plan over two years and came up with a cost of $21-26 billion for up to 80km of new rail.

“Even if that’s matched by a future state government that’s still a $20 billion black hole,” he said.

NSW Labor leader Luke Foley welcomed the announcement, describing his federal colleague as “Father Christmas”.

In the final NSW Labor conference before both state and federal elections, Mr Shorten also promised $300 million in federal funding for parking facilities near public transport amenities.