I play sport for serious and fun purposes, and training is basically 7 days a week. Dont know if this would apply for everyone else, but as long as you take the attitude that you have on the sporting field onto the table, you should be fine, regardless of the amount of HW you get. Its about an objective realisation about what needs to be done straight away at any given moment, ie dont be too rigid in your activities/studies(unless you work well with schedules).
Also, you shouldnt be stressed from school, only tired, maybe. As long as you actually learn the stuff and integrate it into your understanding of the world, most of the HSC courses are quite easy and also bordering on fun(ok maybe thats taking it a bit too far). Finally, you shouldnt make any goals for your HSC that are anything below 100%. You aim for 100% for every assessment you do, and regardless of whether you end up with 100 or 50%, you brush it off as if it didnt happen, since its pointless to dwell on anything in the past. Ive found that this attitude helps with greater performance at school, probably because psychologically you arent restricting yourself to parameters.
I just read over what I wrote, and Geez, I talk a lot of shit. Anyway, hope you've understood what I've said, because it certainly helped me heaps when I learnt these things.