The Pirate Bay Web site, which claims to be the world's largest Bittorrent tracker, is a site that allows visitors to share and download potentially illegal files that include music, films and software.
A spokesperson from NAB told ZDNet Australia that the bank did not advertise on The Pirate Bay site.
"I can say without a shadow of a doubt that we are definitely not advertising on that site. We are very limited and selective with our online advertising and our advertising is limited to business and consumer sites such as ninemsn and Fairfax".
However, following a request from ZDNet Australia, antivirus firm Sophos investigated the Pirate Bay site and located banner advertisements for both the NAB and St George Bank.
Sophos confirmed that the advertisements on the site, which proudly publishes and ridicules threatening legal notices it has received from the lawyers of companies such as Microsoft, Apple, Warner Bros and DreamWorks, were legitimate and led to the official NAB and St George Web sites.
When confronted with this information, the NAB spokesperson said further investigations would be required.
"We can certainly investigate that but we are certainly not advertising on that site," the NAB spokesperson said.
St George was not immediately available for comment.
ZDNet Australia has forwarded the necessary links and screenshots to the banks in order to aid their investigations.