How to get second chance at pursuing honours/PhD if your undergraduate grades are poor in Australia?
Regarding your hypothetical argument about getting accepted to honours and good grades there not mattering: if you did get accepted and get great grades in the so-called honours program, It would almost certainly help you far more than by just averaging up your GPA. Universities will notice the upward trend and some will be willing to believe you have turned things around.

More generally, no matter what you do next, poor grades say two things, both of which you need to overcome:


You have poor study habits, hence you'd probably fail out of grad school by doing more of the same, because a "C" average isn't acceptable in grad school.
You didn't learn as much of the material as you should have, and are now behind everyone else who got better grades. And now you will be in over your head in graduate courses which build on those undergraduate courses.


You can address #1 by showing a pattern of improvement in taking new classes and doing well. For #2 you will need to basically learn the material in a second try. If you had non-existant study habits in school with all the support and pressure on you, I don't have too much hope for self-study. I'd suggest finding a way to retake some key classes.

I'd also note that machine learning may be very different than what you expect coming from a computer science background. It is a heavily mathematical subject, where the programming is the easy part. So you may be gaining some edge by focusing on a topic that is more interesting, but you're also doubling-down on what is likely the hardest component of your studies.