WIKILEAKS is the target of an ''unprecedented'' US government criminal investigation, Australian diplomatic cables obtained by the Herald reveal.

The cables also show the Australian government wants to be forewarned about moves to extradite Julian Assange to the United States, but that Australian diplomats raised no concerns about him being pursued by prosecutors on charges of espionage and conspiracy.

WikiLeaks founder Julian Assange talks during a news conference in central London.

The cables, released under freedom of information to the Herald this week, show Australian diplomats have been talking to the US Justice Department for more than a year about US criminal investigations of WikiLeaks and Mr Assange.

While the Justice Department has been reluctant to disclose details of the WikiLeaks probe, the Australian embassy in Washington reported in December 2010 that the investigation was ''unprecedented both in its scale and nature'' and that media reports that a secret grand jury had been convened in Alexandria, Virginia, were ''likely true''.