The country's big cities have become "disposable" and risk losing their character as Australia's propensity to import fly-in-fly-out architects intensifies, two Sydney designers warn.

"Think of projects in Sydney – like Darling Harbour, or the stadium – where the life of the public project is lucky if it survives 30 years before it is erased and started again," Laura Harding of Hill Thalis Architecture told The Australian Financial Review.