The Turnbull government is under pressure to allow foreign airlines to operate as many flights as they want to the planned new Western Sydney Airport in order to boost its commercial viability.

Classifying it a "regional airport" under international agreements will also give it a leg up against Sydney Airport, which investment banks now rate a low chance of constructing the new airport at Badgerys Creek.

NSW Opposition leader Luke Foley has demanded the federal government confirm that the new airport will be exempt from the restrictions imposed on foreign airlines which fly to Australia's major airports such as Sydney's Kingsford Smith.

"This is a way to support the airport at no cost to the taxpayer," he said in a letter to federal Urban Infrastructure Minister Paul Fletcher.