Extra-curriculars don't guarantee school entries, they just support your presentation. They serve as a form of creditation to paint a picture of you, who you are and your traits. If you had in your resume that you played cricket and you won 3 championships, it would indicate that you were hardworking, diligent, responsible and sporty which would create interest in you as a person as you are more distinctive than the average candidate. By no means is doing extra-curriculars alone is good enough for entry into schools let alone selective schools, if you're doing poorly in performance but you're great at other activites, then there's no real incentive to select you.
Extra-curriculars assist in making you stand out/distinguish yourself from a sea of candidates and it helps paint a picture of who you are as a person and student. That is why it is important to make sure you stick to 1 or 2 activities and excel in them (if you can without taking away from your school time) so that you have reputable and high positions such as a leadership role or some form of achievement such as winning a tournament etc. It highlights your qualities and traits. You can also use them as a point of reference in your interview, to back up your claims of yourself and even get referees to comment on you which helps. They help a lot but by no means guarantee entry, they just paint a picture and when you're one of many candidates, that's crucial. However if you're just naturally talented at school and your report is stunning, then you should be just as noticeable and distinctive.
Its about standing out in a large pool of candidates, find something you can put down in your application that makes your name stick and get you an interview. This thread is extremely large so I recognise you all are very eager and determined to get into a school and I appreciate such effort and envy it, really good you're having it at this stage. Just know if you get it on or not, it doesn't really matter as long as your school is pretty average and you have the determination and grit, nothing can stop you from doing good in the HSC except yourself.
All the best once again.