The Turnbull government won't say if a rail link to Western Sydney's planned airport will be ready when the first plane is due to take off in 2026.
Construction of new roads to help passengers reach western Sydney airport is officially underway, but doubt remains over when a crucial rail link will be built.
Prime Minister Malcolm Turnbull on Monday turned the first sods of the Northern and Bringelly Road upgrades with a golden shovel, before hosting a joint press conference with Treasurer Scott Morrison and Urban Infrastructure Minister Paul Fletcher.
The projects, which will widen some sections of the roads to four lanes in both directions, are part of a $3.6 billion western Sydney infrastructure plan.
"This infrastructure development, together with the western Sydney airport which we are going to build, is going to add tens of thousands of jobs to western Sydney," Mr Turnbull told reporters in Orchard Hills.
"It is vitally important that sources of economic growth and activity are developed here."
But despite pledging the roads would be open by 2020, the federal government remained tight-lipped on whether a rail link would be operating by the time Badgerys Creek opens in 2026.
Mr Fletcher told AAP the coalition wouldn't make any promises until it had seen a scoping study - conducted by the federal and state governments - due to be finalised in the coming months.
It will outline the rail link's optimal route, cost, timeline and funding methods.
"Once we've received it we'll be having more to say about the way forward," Mr Fletcher said in western Sydney.
Federal Labor says Badgerys Creek will be a "complete joke" if it opens with no supporting rail link.