Customers of the Commonwealth Bank, St George Bank and CUA are among those hit by the fraudsters.

Card skimming involves information being illegally copied from the magnetic strip of a credit or debit card.

Once the card has been skimmed, a fake card with the victim's details is created to carry out fraudulent transactions.

Police from the State Crime Command's fraud squad have joined Wollongong detectives in investigating the thefts, but neither would release details about which ATM or EFTPOS machines were involved.

The Thirroul Commonwealth Bank ATM was out of order yesterday with a sign attached saying it was experiencing "technical difficulties".

Austinmer's Nick Boers was among those who discovered illicit transactions had been made on his credit and debit cards.

"I went to get fuel this morning and both cards were declined which didn't make sense because I knew there was money in both," he said.

Mr Boers said a total of $2300 had been taken from his accounts in cash advances between Sunday and about 5am yesterday.

He was at the Commonwealth Bank yesterday morning, filing a dispute claim for the transactions.

"They said I'm the tenth customer this morning (affected)," he said. "I heard another guy say he lost $6500. I was devastated, I work too hard for it."

An Austinmer woman said several members of her family had fallen victim to scammers.

"My husband's and my accounts, which have different cards, lost a total of $1300," she said. "My sister also had $1400 taken."

The woman said when she was reporting the fraudulent transactions to St George Bank, she was told hundreds of customers might have been affected.

"The lady beside us at the bank had $10,000 taken," she said.

It is understood that the transactions were made in Canada, India, Kogarah and Rockdale.

Detective Sergeant Steve Davies said customers concerned they might have been a victim of a card skimmer, should contact Wollongong police on 4226 7899.

A Commonwealth Bank spokesman said the bank would try to contact any customers that may have been affected.

He said if customers were concerned about their account security they could change their PIN.