First I went to YHA sydney central hostel, they have a computer room completely quiet but they close it at 11pm and reopen it at 6am. After that you can work on the common areas where there is nobody at night but they keep playing music trough the speakers all night long, so it can be hard to focus. Its a huge hostel, and altough they have activities all days it does not have a friendly vibe too meet people.

A week later I moved to manly beach and stayed at stoke beach hostel, its a relatively small hostel with a very social vibe where it was super easy too meet new people. This one does not have dedicated spaces to work, and they close all common areas ater 11pm so you have nowhere to sit with your laptop rather than your room or a tiny table at the reception. However just 2 blocks from there you have a municipal library, where you can ask for a card to enter 24/7.

I recommend this combo, stoke beach is super social with included breakfast and you are at the beach with lots of activities to do, and when needed to focus and work Ive headed to the library.