The communications regulator says a computer malfunction caused harmless Bill Henson images to be added to its blacklist, rather than an incorrect assessment of the content.

Online users' lobby group Electronic Frontiers Australia said this painted an even scarier picture of the Federal Government's forthcoming internet censorship regime because it was now clear that not only human error caused sites to be incorrectly added to the blacklist but computer errors as well.

"It's already hard enough to imagine that a hand-edited blacklist could stay accurate and keep up with the internet, but now if their computer system is just so unreliable that sites are getting on there by mistake, then we really need to take a step back and look at another solution," said EFA spokesman Colin Jacobs.