A suspected Melbourne Mafia boss's lawyers are seeking access to the confidential sources of an Age investigative journalist in a move that is being refused by Fairfax Media.

A letter from lawyers representing multi-millionaire Melbourne grocer Antonio Madafferi was sent to The Age on Tuesday, claiming that the identity of the source should be revealed as they could also be subject to defamation action.

The Mafia poses an "extreme" organised crime risk to the nation, police reports warn.

Age journalist Nick McKenzie has exposed links between Mr Madafferi and the Calabrian Mafia, and outlined the extent of his reach – including donations to the Liberal Party and hosting two fundraisers – in a series of articles published by Fairfax Media.

Mr Madafferi was recently banned by the Victoria Police chief commissioner from Crown Casino because of his alleged organised crime ties, while the federal police have publicly outlined his close associations with known drug traffickers.