The link in question was to an anti-abortion page containing some pictures of aborted babies. Apparently a member of the forum decided to test the filter by posting a link to the page and then submitting a complaint to the ACMA asking for such a link to be banned, for the purpose of seeing what would happen.
Lo and behold someone at the ACMA must of looked at the page, seen the pictures (I'm sure you can find much worse in any medical journal mind you) and decided that linking to the page was now illegal. So they sent a notice to the forum's hosting provider (bypassing the forum all together) informing them to take the link down within 24 hours or risk being fined $11K per day. The host then contacted the forum admin who obviously didn't want to put this on his provider took down the link.
I initially thought nothing would come of this ridiculous filter idea because it was just so plain stupid and so many people, including most ISPs, are against it. But I'm not so sure anymore.