www.topcoder.com
look at the programming competitions there. you don't have to do them, but the questions are very much based on making good algorithms..although alot of the algorithms require you to know sorts that they don't cover in the HSC. the best thing about the website however, is that it does (in the forum sections) cover some of the sorts we do use, and it has programming questions that require you to make algorithms based on these (somewhere, you've got to look under certain type of questions).
Although, in the end, I'd say just keep practicing past papers, they're probably easier and better for algorithm practice.
