The Sydney gateway connecting WestConnex to the city's airport and port will be one of the last pieces of the $16.8 billion project to be built because it is "logical" for it to coincide with completion of the M4-M5 motorway link, an inquiry has heard.

The so-called gateway was one of the original justifications for WestConnex but there remain few substantial details on its likely final shape years after the government committed to the controversial motorway project.

Salary issue was "clouding conversations around WestConnex": Dennis Cliche. Credit:Nic Walker

As part of a broader transport plan, the government is considering duplicating the rail line that runs from Port Botany and past the northern boundary of Sydney Airport to the city's west.

That will have a bearing on the shape of the Sydney gateway and is likely to require parts of the existing rail freight line to be shifted which, in turn, could lead to compulsory property acquisitions and a major escalation in the cost.