Its just a pity they can't arrest the terrorists.
Which terrorists in Australia? What terrorist attack on our home soil are you referring to? As far as I'm aware there has been only one successful terrorist attack in this country since the eighties (on an off shore Australian embassy) and only a couple of attempts before then. While successful, the Bali bombing was not on Australian soil.
Idiots who make jokes are just a useful stats building resource. Fear of terrorism is a different matter; emotive, media driven and impossible to mitigate. Maybe the would be terrorists have already been arrested, maybe they never existed. Either way, we have had no terrorist attacks, so criticizing the national security organizations for not arresting terrorists seems a little unfair.