Which Uni? Macquarie Uni for example doesn't offer honours per say (except for Law but that operates differently)
This. I believe Computing Science at UTS has mandatory honours for example, as it's intended for people interested in research.
At UNSW, as blyatman explained, I believe you would pick to do honours much closer to when you would be up to that stage, and there would be a minimum WAM requirement etc. This is probably the case for the majority of courses and universities.
I don't know how common it is for computer science, but apparently you can even do honours at a different university to where you studied your undergraduate course - but i've only heard about this for competitive honours programs where you really need honours to pursue the main career path (e.g. psychology).