Four men working on a redevelopment site at Sydney's Villawood Immigration Detention Centre have been found to be illegal immigrants.

A spokeswoman for the Department of Immigration and Citizenship says the men are Chinese, Malaysian and South Korean nationals.

They were found to be working without valid visas during a compliance operation at the south-western Sydney facility last week.

Three of the men have been detained inside Villawood while their claims are being processed.

The fourth man has been granted a bridging visa.