Australian police said they were investigating the theft of some 500,000 credit card numbers which resulted in Aus$25 million (US$26.2 million) worth of fraudulent transactions.

Authorities were unable to say whether the investigation was confined to Australia but noted that "the borderless nature of this crime type poses new and unique challenges for law enforcement".

"The Australian Federal Police can confirm it is currently investigating a series of merchants whose individual computer systems have been compromised," a spokeswoman said.

"The compromise is believed to have involved approximately 500,000 credit cards and resulted in more than $25 million in fraudulent transactions."

Reports said eastern European hackers were responsible, although police did not confirm this, only saying that international and private sector cooperation was critical to its ability to target this type of fraud.

"This investigation demonstrates the importance of the AFP's close working relationship with its international law enforcement counterparts, private industry and the financial sector to combat this crime type and bring those responsible to justice," the spokeswoman said.

"As this operation is still ongoing, it is not appropriate for the AFP to provide further information at this stage."