Prominent activist group GetUp has been unwittingly caught up in card-skimming scams targeting almost two million Australian-based cards a year.
The scams had involved several fraudulent donations being made to the group but returned within hours, a GetUp spokesman said.
In a statement, he said the fraudulent website donations were detected on a "small number" of donations.
"We immediately and proactively worked with financial institutions to refund the money," he said.
The spokesman said the organisation worked with police to investigate the fraud but did not reveal how many cases had been detected or how much money was involved.
"We understand through discussions with the police that the fraud occurs after vendors skim card details in places like restaurants and taxis," he said.
"Police informed us that card numbers are used to make donations to organisations online to estimate card limit so as to increase their value on the blackmarket."
According to the Australian Payments Clearing Association, there were 1,895,432 fraudulent transactions from credit and debit cards and cheques in 2014-15.
Those payments topped $412,282,588 and made up about 0.027 per cent of all transactions.
The GetUp spokesman said his organisation's website used "256-bit cipher enterprise grade encryption" and was fully compliant with enterprise standards.
"This is a national problem and GetUp takes its responsibilities as an online merchant to mitigate the impact on cardholders very seriously," he said.
