Called "cash trapping", offenders insert an implement similar to a fork to an ATM during a small withdrawal transaction that jams the cash dispenser.

Melbourne's County Court heard that the fork then "entraps" the money from a second transaction, usually the daily limit, after the ATM software records a dispensing error on the first one.

The "forking" scam was recently revealed to a judge who heard that four men had conspired to defraud a total of $184,920 from 248 mostly suburban Melbourne ATMs between December, 2012, and March, 2013, using "card skimming" and "forking" techniques.

Prosecutor Peter Pickering told Judge Liz Gaynor that police began Operation Whitebait after learning that illegal transactions were being committed on ATMs in Europe by Romanian nationals.

Mr Pickering said the response of investigators – from the Victoria Police fraud and extortion squad – followed allegations that similar transactions in Melbourne were targeting local banks and branches.

Card skimming, he said, involved devices attached to ATMs, typically surreptitiously over the machine's card reader, which could read encrypted information on the card, while a small camera recorded the customer's PIN.

Judge Gaynor described as "nightmare stuff" peoples' accounts being attacked which placed the entire community at risk and undermined public confidence in the banking system.

"It's the nightmare stuff people are constantly warned about when visiting ATMs," she said.

Four Romanian-born men, Florinel Camara, 25, Antal Ciprian Donka, 37, Christian Georges, 53, and Velentin Dumitru Anania, 30, each pleaded guilty to conspiracy to defraud, while Camara and Georges pleaded to a second identical count.

Mr Pickering said Camara typically made the transactions or inserted the fork while Georges stood watch or obscured the CCTV.

Donka and Anania acted as lookouts or drivers, and while Camara was present on all occasions when cash trapping was done, the others, but not all, were involved on a number of occasions.

Mr Pickering said police had identified other offenders, known as "White Stripe Man" and "Gant Man" from their clothing, but they had not been arrested.

All four were arrested on March 12, 2013, and bailed, he said, but Camara and Georges reoffended by committing identical offences in August that defrauded $25,600.

James Yanolatos, for Camara, said his client was "gently persuaded" by Georges and taught by him how to engage in the activity and was supplied cocaine, but never drove to any location and handed all money to Georges.

Mr Yanalatos agreed with Judge Gaynor that it was his client's case that he was "entirely under the sway" of Georges and was, ultimately, incapable of resisting him.

Chris Farrington, for Georges, said his client "absolutely" rejected Camara's version and also that he supplied Camara with cocaine and had played a lesser role than Camara.

A former professional soccer player, Georges' motivation for the offences was "simply financial", he had reoffended for financial reasons and "acknowledges the wilfulness of his conduct", said Mr Farrington.

Will Barker, for Donka, said his client was a "man of good heart, but for this" (offending), which he had "turned a blind eye to on purpose".

Anania's lawyer Camille Woodward told the court Camara introduced her client and Donka to a "party lifestyle" of drugs and alcohol and he "jumped in with both feet" and went on a "rampage".

Ms Woodward said he did not "exactly" know what was going on, but he "willingly shut his eyes" to the offences.

In his sentencing submissions, Mr Pickering urged Judge Gaynor to be "sceptical" about how each man had described how they got involved.

He noted that each offender had pointed blame in different directions, but $184,000 was still missing and it was a conspiracy between all of them.

Camara had been in and out of Australia for years, Georges arrived in 1980 while lifelong friends Donka and Anania arrived in October, 2012 for a holiday.

In her sentencing remarks on Friday, Judge Gaynor was satisfied that Camara and Georges played leading roles in the conspiracy, then Anania with Donka with the "least proved" involvement, but she was "cynical" about each mans' instructions to their lawyers.

"Each of you has sought to lay blame for your offending on others," she said – Camara onto Georges, Anania and Donka, and Georges, Anania and Donka onto Camara.

Judge Gaynor categorised such conspiracies as striking at the "security and safety of the way in which the vast majority of people carry out their ordinary banking".

"It exploited a financial system which has the capacity to affect the everyday lives of millions of people," she said. "In protection of it, a court's response must be one of particular sternness with the imposition of sentences designed to deter others from like activity, to mark the seriousness of this offending and to punish those offenders who engage in it.

"The possible consequences of your offending, if it had continued on unabated and undetected, could have been calamitous."

Camara and Georges were jailed for six years with a minimum of four years, while Anania was sentenced to three years with a minimum of two years and Donka to two years with a minimum of 16 months.