Motorists face paying two sets of tolls to drive from western parts of Sydney to the city's main airport as property and construction giant Lendlease seeks the rights to build a crucial motorway link from the WestConnex toll road to both the airport and port.

Lendlease, which is riding a construction boom in NSW, has lodged an unsolicited proposal with the state government for the so-called Sydney gateway, extending from a major interchange for WestConnex at St Peters in Sydney's inner west to Mascot.

While few details of the link's likely final shape have been released, the government says the gateway will not be part of the distance-based tolling regime for WestConnex, which is capped at $8.60 in today's dollars.

That means motorists driving from parts of western Sydney such as Parramatta to the airport face paying well above the toll cap if separate charges are imposed on the gateway.