It might help in having an overall knowledge of chemistry but it doesn't mean it will directly help you in HSC. The content is the same in some areas but then the style of questions is also different. HSC there is a lot of writing type questions.
If you want get a better understanding of chemistry and maybe do it in future as a degree then yeah olympiad would be a good experience. However, if you are talking directly related to the HSC you are better off going to a HSC summer course at a tutoring centre or going to a private tutor during the summer time.