Victoria's Opposition Leader Matthew Guy says he did not know an alleged Mafia head would be at a dinner he attended at a Melbourne lobster restaurant, and there was no talk of political donations at the event.

The dinner was attended by Tony Madafferi, a wealthy market gardener and the owner of the La Porchetta pizza chain, who has repeatedly been accused by police in court of being involved in organised crime.

Tony Madafferi has been accused in court of being a high-ranking member of Melbourne's Mafia. ( ABC News: Sam Clark )

Police have banned him from Crown Casino, but the ban is being challenged in court.

Mr Guy claimed he did not know Mr Madafferi would be at the dinner, at the Lobster Cave in Beaumaris in May, until he was introduced to him as "cousin Tony".

Mr Guy said the dinner was organised by Liberal Party member and AUSVEG Victoria executive Frank Lamattina to discuss the Epping fruit and vegetable markets, and there was no talk of political donations.

Mr Lamattina is a long time Liberal member and Mr Guy said he had agreed to meet his family to discuss the fruit and vegetable market.

He stressed that he did not know Mr Madafferi was related to Mr Lamattina.

"There was no talk that I sought donations or received donations from anyone at that table," he said.

"If I was there to hide what I was doing, if I was there in some kind of secret clandestine meeting, why would you do it in the middle of a packed restaurant?"

Mr Guy denied drinking Grange over the two-hour dinner with around a dozen people, saying he rarely drank red wine.

His statement in response to the joint Four Corners-Fairfax investigation, which revealed the pair attended the same dinner, said there were about 20 people at the event.

The dinner took place at the Lobster Cave at Beaumaris in May. ( Fairfax Media: Joe Armao )

Mr Guy has built a strong agenda against the Government on law and order. He said that, in politics, you cannot control everything.

Deputy Premier James Merlino said Mr Guy had serious questions to answer.

"The claims of Matthew Guy … simply aren't believable," Mr Merlino said.

Unlike Premier Daniel Andrews, who would not comment, Mr Merlino said the dinner raised questions about Mr Guy's the credibility to lead a political party.

He said the dinner made "an absolute farce" of the Coalition's law-and-order attacks.