Up to 12,000 people donating to charities through an Australian website have had their credit card details stolen and used for thousands of dollars worth of fraudulent charges.

An international hacking operation targeted the MyCause website, which allows people to create and donate to fundraisers and charities.

Some of the credit card details have been used to make fraudulent transactions in the United States, Europe and the United Arab Emirates.

Website founder Tania Burstin said people who donated in June and July could have been stung.

"Effectively people are doing good. They're donating to charity, so this is very, very vicious," she said.

Ms Burstin said the website's security was industry compliant but said she is now enhancing its anti-fraud protection.

She said she believes the hackers had been specifically targeting MyCause as a charity website.

"Our brand is out there and it's known, as it's also known to the bad guys out there," she said.

Canberra woman Emma Barnes said she lost her friend to cancer.

After donating to help the family, she was hit with a $3,000 charge on her credit card from Walmart in the US.

"It's just terrible, I don't know how they can live with themselves. Frankly it's an awful thing," she said.

Melbourne man Sergio Correa said the thieves used his card to spend more than $1,500.

He said he used the site to help send a team to a sporting competition in China.

"It's pretty annoying and frustrating. You try to do something good for a cause and it backfires on you," he said.

"Moving forward, I'll be more careful where I'll put my credit card details, if I do at all."