French property giant Unibail-Rodamco  is to buy Australia's Westfield shopping mall operator, the two groups said Tuesday, in what would be the biggest-ever corporate takeover in Australia.

The deal values Westfield at US$24.7 billion and would create "a global property leader", they said in a statement.

The future combined portfolio of the merged companies would include key centres like Westfield London, Westfield World Trade Center in New York, Les Quatre Temps near Paris and the Forum des Halles in the heart of the French capital.

The agreement would "create the world's premier developer and operator of flagship shopping destinations", they said.