Police suspect accused Melbourne mafia boss Antonio Madafferi has put out a $200,000 "contract" to murder a man he believed was providing information to The Age about the grocer's activities, a court has heard.

Investigative journalist Nick McKenzie, in an affidavit filed with the Supreme Court, claims a detective had warned him about the alleged murder contract in mid-2015 and that Mr Madafferi was also looking at ways to put the journalist under surveillance.

Antonio Madafferi, pictured left, is seeking access to confidential sources of Age journalists. Credit:Jason South

"I believe that one reason Tony Madafferi may wish to have me followed is to see to whom I speak to about his activities so he may silence these persons," Mr McKenzie said in his affidavit, which was released by Justice John Dixon on Friday.

Mr McKenzie said the detective revealed Mr Madafferi was suspected of having put out a "contract" to arrange the murder of a person the grocer suspected of assisting Fairfax Media in researching articles about him that he claims are defamatory.