A split is developing in state coalition ranks over plans to develop Sydney's second international airport at Wilton north of Mittagong.

Premier Barry O'Farrell says he opposes plans to built a new airport anywhere in the Sydney basin, including Badgery's Creek which was the first choice, and now new federal government new  favourite,  Wilton.

However some members of the state Liberals coalition partners, the Nationals, appear to have differing views.

The issue will come to head at a National Party conference in Bowral this weekend, where members of the Dubbo and Parkes branches, will raise the airport issue.

They believe an airport should be built in the Cumberland basin, which includes Wilton.

Mr O'Farrell continues to push the line of expanding Canberra airport, and then building a high speed rail line through the highlands to Sydney.