Victorian Opposition Leader Matthew Guy is taking the extraordinary step of referring himself to the state's corruption watchdog after dining with an alleged Melbourne Mafia boss at a lobster restaurant.

It comes after a Fairfax Media-Four Corners investigation revealed Mr Guy was at a dinner earlier this year that was also attended by alleged Mafia head Tony Madafferi.

Mr Guy said he was confident he had not done anything wrong in attending the dinner.

"So confident am I of my conduct in this matter — the fact that I sought or asked for no political donations, the fact that I went along to a meeting to discuss the matters which I outlined to the media this morning — I would like to myself refer this matter to have it fully investigated," Mr Guy said.

He said he believed referring himself to the Independent Broad-based Anti-corruption Commission (IBAC) was the "right thing to do".

"I believe that will clear up any matters in relation to probity that some have asked or insinuated around today," he said.

"This is a very clear, forceful, proactive way of saying I believe I've done nothing wrong, and if someone thinks otherwise, well, it will be up to them to go forward and to prove that or to state that."

Mr Guy made the announcement after secretly recorded phone calls brought into question Mr Guy's claims that he had no idea he was meeting Mr Madafferi at the restaurant.

A Liberal Party figure who organised the dinner, Barrie Macmillan, was recorded saying he had personally briefed Mr Guy's office about Mr Madafferi's presence at the dinner, and stressed the need to keep the meeting confidential.

Mr Guy says he was not personally told Tony Madafferi would be at the dinner. ( Fairfax Media )

"I had to forward the names to [Matthew Guy's private secretary]. I had to do it all through [her]. It was all kept fairly hush hush," Mr Macmillan, who also attended the dinner, said in the recording.

But Mr Guy said he had not personally been told who would be attending the dinner.

"It has been alerted to me that a phone call to my office was made with … a few names, one of those being Antonio Madafferi, that was not passed on to me," he said.

"I accept that was a name that was passed on to my office, it was not passed onto me. Antonio Madafferi was not a name that rang any bells for investigation."

Mr Guy said the process in his office needed to improve.

Mr Madafferi is a wealthy market gardener and the owner of the La Porchetta pizza chain, who has repeatedly been accused by police in court of being a high-ranking member of Melbourne's Mafia.

In an affidavit filed in court in June to support Mr Madafferi's ban from Crown Casino and all Victorian racetracks, police said they held "substantial intelligence" indicating that Mr Madafferi had "substantial and close involvement with serious criminal conduct including drug importation, murder and extortion".

Mr Guy has cultivated a "tough on crime" persona as Opposition Leader. But he has previously been warned about associating with alleged Mafia figures, after he claimed as planning minister he had "unwittingly" become the star attraction at a 2013 fundraiser hosted by Mr Madafferi at his Docklands venue centre.

He told the gathered media that he had never met Mr Madaffari before the dinner at the Lobster Cave earlier this year, and did not recognise him when he arrived.

"Until I met him in April I didn't know what he looked like, which is why I asked Frank [Lamattina] what Tony's last name was," he said.

Mr Guy also stood by his earlier comments that 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.

"I don't know the relationship between Frank and Barrie. Frank Lamattina is a contact that I've known … for a long time. Frank Lamattina organised the dinner. They're Frank's family, not Barrie's family," he said.

He said the recorded telephone conversation would form part of the body of evidence that should be referred to IBAC for investigation.

The Lobster Cave in Melbourne's south-east, where Victorian Opposition Leader Matthew Guy attended a dinner with Tony Madafferi. ( Four Corners )

Deputy Premier James Merlino accused Mr Guy of using IBAC as a distraction, and Mr Guy had "failed a test of character" by having dinner with Mr Madafferi.

"Matthew Guy knew exactly what he was doing, and he knew it was wrong and he's been caught out today," he said.

"It's not an issue in terms of his self referral to IBAC — it's an issue in terms of his character and his judgement."

Mr Guy has called on Premier Daniel Andrews to guarantee Labor MPs have themselves never met with Mr Madafferi.

Mr Merlino said he never had, and was not aware of any of his colleagues ever doing so either.

'It's quite offensive'

It is understood Mr Macmillan organised the event with Mr Lamattina because Mr Madafferi and his relatives are long-time Liberal donors, and because Mr Lamattina had, at an earlier fundraiser, been promised a private dinner by Mr Guy.

Mr Guy strenuously denied that he went to the dinner to solicit donations to offset the Liberal Party's difficult financial position, after losing some of its traditional donors after a spat with party officials.

"It's been inferred that I went there seeking money. That is completely and utterly false, it's quite offensive.

"I did not go there for that purpose, I went there for the purposes I outlined."

Mr Guy said the Liberal Party's State Director could not find any evidence that the party had received donations from any of the attendees at the dinner.