Don't know what advice you're looking for - all I can say is, 6 subjects in year 12 is definitely too much. But also, if all you're aiming for is science at unimelb (an ATAR requirement of usually just 86), then a 6th subject at most will only add up to 1 ATAR point - it's not necessarily going to add as much as you think it will, and it's not the end of the world if you don't get it. At the same time, why be in such a rush to graduate? Take it from experience - uni can be a socially much harder place to deal with if you're running around below the age of 18. Even if you're not interested in drinking, there are many times people will want to meet up at a bar, there's many camps you might be interested in going to but they won't let you because you're underage, etc. Socially, uni is a better experience when you're of age, regardless on your opinion of alcohol and whether or not you plan on having any.
On top of that, if you're doing this because everything's too boring, you could instead negotiate a half-half situation. Do half of your subjects as 1/2 next year, then when you're in year 11, do half 1/2 and half 3/4, and then in year 12, just study 3 subjects. You still get most of the acceleration done, and you finish high school so that you're 18 when going to uni. Plus, this way you get 6 subjects - hell, you can even jump on 7 if you really want.