Hi there! I'd be glad to help you out and provide a bit of info about my experience with scholarships. I was offered 2 early offer scholarships from the University of Sydney and also an Academic Excellence scholarship from Western Sydney University.
The first thing to scholarships is to know when applications for them open. Some are available very early in the year while others are available after the HSC is over. For example, UNSW's Co-op scholarship opens super early and you're given a couple of months to complete the application since it's SUPER long. On the other hand, some of WSU's scholarships open AFTER HSC results are released. USYD has scholarships open throughout the second half of the year like E12, Sydney Scholar etc. Make sure you're aware of application dates and start early!
Regarding extra-curriculars, many scholarships nowadays look at a "whole" student, as in, they look beyond just studies and academics and look at what you've contributed to your school, your community etc. They want to know what you're passionate about and enjoy doing in your spare time. I believe it's really important to not only have good academics but to also participate in extra-curricular activities that you like or may be relevant to the course you'd like to pursue. I had my sights set on Commerce in university and a majority of my activities were related to that field. I participated in school competitions like Mock Trial and Mock Mediation, was a part of committees within my school, and was a State Grand Finalist in a national Business Studies competition. I also volunteered outside of school in my spare time for charities I was passionate about and those whose values or visions I wholeheartedly supported. Don't blindly participate in things you aren't passionate about or interested in! Make sure they're about things you like or enjoy.
Many of USYD's scholarships aren't based on application, but more of them choosing the students to offer scholarships too. I received an early offer for E12 but through my application, they also brought it across to another scholarship within my chosen faculty, so try to make your application stand out from other applicants!
For the academics side of things, I won't lie, they will play a factor as well. USYD's E12 scholarship asks for a copy of your latest school report and UNSW's Co-op scholarship application requires you to input your marks and ranks during Preliminary and HSC year. WSU's Academic Excellence scholarship is based on, well, academic excllence (duh) Throughout the HSC, my marks weren't 100% superb and they don't have to be! I got average marks and some subjects I did really well in while others I didn't do so well in. Like I said before, most universities want to look at the "whole" student, so if you ace those questions during the application process, your grades shouldn't impact too much.
Good luck with your scholarships and I'd be happy to help if you have any other questions (: