i think a tutor in year 11 is good ground for your studies in year 12 if you seriuosly want to do well. you need to get in the habit of studying and you need to understand your year 11 phys/chem/bio stuff really well in year 12, otherwise you'll be wasting much of your time in year 12 trying to understand it. i mean, i know some people who did really well and went by in year 11 without a tutor and theyre still not getting tutored and doing well. its up to you as person. but if your like an ext 1 maths student who wants to do ext 2 maths, then it is ideal to get a tutor so you are guarunteed a spot for ext 2. IMHO, i dont think you need a biology tutor. i went through year 11 with a biology teacher who couldnt teach and i managed to come 4th in the grade. but you'll find that many concepts in year 11 chem will build on in year 12.
in general, a tutor will help "motivate" you, depending on your tutor. make sure you get a tutor that will pay attention to you and knows what theyre doing. theres plenty of people out there who claim theire tutors but really, they just reading off what theyr wrote on the board or just reciting the text book to themselves.
my 2 cents