I’m personally with Vodafone so it’s easy to recommend.
There is also a Vodafone store 2 doors down from Bondi Beach Backpackers so problems can be fixed easy.
Other options are Optus and Telstra plus a ton of online ones.
Free Vodafone and Optus sim cards on reception wall, just help yourself.
You can get credit at the post office or the Vodafone shop or online
I hope you will find this helpful.
Rich @ Bondi Beach Backpackers