A proposed $3 billion will be pumped into building roads to support the construction of Sydney's second airport at Badgerys Creek.

The federal and NSW governments are expected to announce the roads package on Wednesday.

The project will span over a decade with some work likely to start within a year, the ABC reports.

Most of the money will be spent on connecting the M7 motorway to the site, while some will go towards smaller local streets.

The commonwealth will contribute 80 per cent of the public funding while the NSW government will make up the rest, with the added responsibility of footing the bill for any connecting rail links.

The proposed $3 billion contribution is designed to head off concerns about increased congestion and will be included in the May budget and extend beyond the four-year budget cycle.