International crime syndicates have swindled more than $4 million from Victoria's public transport system, using stolen credit card details to buy Myki cards which they then sell on the black market.

Key points: Stolen credit card details have been used to buy Myki cards, which are then sold

Stolen credit card details have been used to buy Myki cards, which are then sold The fraud is now the subject of an international police investigation

The fraud is now the subject of an international police investigation Victorian commuters assured their Myki accounts are safe

Victorian commuters assured their Myki accounts are safe PTV confirms it refunded $4.2 million to international credit card holders

Public Transport Victoria (PTV) confirmed it had refunded $4.2 million to international credit card holders who had Myki purchases charged to their accounts over the past 18 months.

PTV chief executive Mark Wild said the fraud was the subject of an international police investigation, but he assured the state's commuters their Myki accounts were safe.

"It's not commuter's money, definitely not. It's nothing to do with people's personal Myki money or their own credit card details," Mr Wild said.

"It's fraudulent activity, mostly by stolen international credit cards.

"We came under significant attack by these international fraudsters in the late part of 2013, early 2014."

Mr Wild said the fraud was related to the card's payment system and had nothing to do with Myki itself.

He said while PTV had extensive fraud protection in place, this kind of "sustained attack" was something many online retailers had to put up with.

"We're just the same as every online retailer. We've got to remain very vigilant to this," he said.

"It's organised, it's a constant threat, but I'd like to reassure everybody it's nothing to do with people's personal details or Myki money.

"We first detected this over a year ago and we've successfully worked with all the financial institutions to such an extent that we've pretty much eradicated it."

But he said the money repaid to the credit card holders had come out of PTV's own pocket.

"I can't say too much about the actual circumstance of the crime itself because the police are investigating it, but certainly it's a loss to PTV and our customers overall," he said.

System needs 'to be made easier' for commuters

Mr Wild also urged people to make sure they were purchasing cards from Myki machines and authorised sellers.

"If people see Mykis for sale on Gumtree or eBay, Mykis that aren't from authorised retailers, you can pretty much be assured that they've been obtained in fraudulent means," he said.

Victorian Premier Daniel Andrews said he was confident PTV was doing all it could to stamp out fraud.

He joined calls for commuters to buy Myki cards from authorised dealers.

"If it looks dodgy, it almost certainly is," he said.

"You get what you pay for. Buying a fake Myki card, a dodgy Myki card, where somebody else has paid for it through their credit card being ripped off, that's not a good deal for anybody."

Opposition public transport spokesman David Hodgett said the Government needed to fix the Myki ticketing system.

"This is a distribution problem. It gives people the opportunity to top these cards up and sell them through unauthorised outlets," he said.

"When I talk to people about the system and their frustrations and their anger at it, they say they can't top up online, they can't get tickets.

"Labor needs to make top up easier for commuters, make access to ticketing easier, so this problem can be prevented."