You see it a million times at this time of the year where people say "I did terrible throughout the year, but if I ace my externals can I get a decent ATAR". Honestly, it is probably the most widely used line on BoS, but it really is easier said than done. Old habits die hard and if you have been unmotivated for the majority of the year, it's unlikely that you are going to suddenly change with a month or so to go. Besides, you will have to catch-up on what you have missed which will make your workload even more significant than someone who has put in the work throughout the year. The HSC is about consistency and most people who do well have been consistent throughout the whole year. It is very rare to find people who have bummed around for the whole year and then suddenly come up good in the externals.
I'm not saying that it cant be done and I'm not saying that people should give up. However, you have to be realistic - it is extremely difficult to jump from doing nothing to putting in the enormous amount of effort required to overcome a nearly a years worth of doing poorly