After a half-century of abortive studies, plans and statements of intent, the federal government will on Monday sign off on what it says is the final piece of paper clearing the way for construction to begin on Sydney's second airport at Badgerys Creek.

Prime Minister Malcolm Turnbull and Infrastructure Minister Paul Fletcher will sign off on the final version of the airport plan and declare construction ready to begin.

"All the approvals are now in place," the Prime Minister said in a statement. "This is the most significant step taken by any government on this project.

"We can now get on with building the airport".