Five days after card-holders first realised their bank accounts had been robbed, the breadth of the fraud was revealed by police yesterday.

Detectives have received reports from 65 victims who have discovered tens of thousands of dollars missing from their accounts since Sunday.

However Detective Senior Constable Michael Bugg said he understood there were at least another 200 victims who had only reported the fraudulent transactions to their banks, not police.

He said there were also people who might not yet realise their accounts had been tampered with by the scammers.

"We're working with the banks at the moment, trying to collate all the information," Snr Const Bugg said.

"There are a large number of victims and we have to confirm where the compromise point was and the time it was."

The compromise point or points are the ATM or EFTPOS machines which would have had skimming devices attached to them to illegally copy victims' card information.

"We're still unable to identify any one compromise point," Snr Const Bugg said. "We're looking at a couple of different sites."

In these types of skimming operations, Snr Const Bugg said a group of fraudsters was usually responsible rather than a lone operator.

"Because of the amount of transactions and the different locations, it's more than likely to be several people," he said.

The Illawarra Mercury understands fraudulent transactions have been made from Canada, India, Campsie, Kogarah and Rockdale.

"It's not uncommon for people to come into an area and set up these devices and leave them and come back to collect them a month later," Snr Const Bugg said.

Fraudsters could pose as EFTPOS or ATM technicians and attach their skimming device to a legitimate machine.

Scammers had also been known to break into a store, replace the EFTPOS machine with their own device, leave it for a period, then break in again and remove it.

"Hopefully by the middle of next week, we should have all the information from the banks and we can determine compromise points," Snr Const Bugg said.

"We're in the process of obtaining the CCTV from all the transaction points in Sydney."