ABC said the email addresses and some personal information of more than 40,000 people had been compromised.
A Twitter user named Phr0zenMyst claimed it had been carried out as part of a hacking mission named #OpWilders.
"ABC hacked for giving a platform for Geert Wilders to spread hatred #OpWilders - database leaked!" tweeted Phr0zenMyst.
The hacker exposed the details of viewers who had logged in and commented on the website of a 2010 programme called Making Australia Happy.
It is not clear if the hacking attack is connected to any group, such as the anarchic Anonymous, which has launched similar politically-motivated attacks in the past.
ABC said it would contact the people whose information had been disclosed in the attack. The website in question has now been shut down.
A spokeswoman added: "At this stage, we are still investigating the details of the breach.
"However, we do know that it has exposed the name, username and a ... version of the password that audience members used to register on the programme website."
Mr Wilders' anti-Islamist views have often proved controversial and the 49-year-old had spoken to ABC's Lateline programme ahead of his speaking tour of Australia.
There have been protests outside some of his events, including one in Melbourne where police had to break up scuffles.
In the past he has called for the Koran to be outlawed and has campaigned to end what he called the "Islamification of the Netherlands".
In 2009, Mr Wilders was initially banned from entering the UK on public security grounds, but the ban was later overturned.
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