Millions of Australians' Centrelink and Medicare files are being handed over to a United States multinational corporation as the federal government closes filing warehouses around the country.

A record management unit in Darwin has been closed by the Department of Human Services while a warehouse in Adelaide will shut next year. Storage units in Townsville and Tasmania are open but "under review".

Warehouses hold decades worth of Medicare and Centrelink files. Credit:Peter Braig

The warehouses hold decades worth of Medicare, Centrelink and Child Support Agency files.

Human Services says the role of US-based Iron Mountain in managing the records will grow as each storage unit closes.