No--it won't be as hard as those competitions. Why? Those competitions are designed to test...actually, I'm not sure what. But the HSC--you have a set of syllabus dot points and/or skills (for maths) and/or essays to learn. That is all. It is simply a memorising game.
e.g. I have only ever gotten one HD in my life (that was for writing) and I didn't get band 6's for either Advanced or Extension 1 English, and I got Participation for Science one year and between Bio and Chem, got an average of 89.5. Also, for a more consistent example--have always been 'credit' in Maths (and got lower 'credit' every year from Year 8 onwards), and got 94 for both 2U and 3U Maths.
Anyway, as for your question of band 6's--consistent study and knowing how to answer the question for sciences--your verbs! of 'discuss', 'evaluate', etc, plenty of practice questions for maths, and knowing how to twist your thesis to suit the question for English. It's only as hard as you make it. Anything else--not sure, but I'm sure others can help here!