It wasn't modified at all. It was this cartoon:
http://www.abc.net.au/mediawatch/stories/popups/060502_s5f1.htm
about which I have mixed feelings. I think it's cheap and inappropriate to compare Israel's actions to the Nazis'. However, of course I don't approve of much of Israel's actions either...
Crikey reported today that it was somebody who works for the Chaser who did it. However, it wasn't an official Chaser prank and they're pretty pissed off at it.
I don't think the cartoon was actually modified at all. Leunig and The Age are being cagey about what the "hoax" was, but the only foul seems to be that it was submitted without his knowledge, and with cover letter written in his name.
http://www.theaustralian.news.com.au/common/story_page/0,5744,18152895%255E7582,00.html