About 400 people have attended a public meeting to protest at proposals to construct Sydney's second international airport at Wilton near Picton.

The Federal Government is conducting a scoping study into the plan, which involves the use of Sydney Catchment Authority land east of the Hume Highway.

Brian Williams from the Stop Wilton Airport group says there is total opposition from locals to the airport plan, while people attending the meeting came from as far away as the Southern Highlands,  Campbelltown and Wollongong.

Guest speaker at the meeting was environmental specialist Ian Wright, who says an airport poses a grave risk to water quality in the area, particularly from jet fuel and run off of chemicals, used in the aviation industry.

Both the state and federal government's have rejected the use of Bradgery's Creek, between Liverpool and Penrith, despite studies showing it is the best location for the new airport.

Premier Barry O' Farrell says Canberra airport should be upgraded and a high speed rail link developed from the national capital, through the southern highlands to Sydney.