G'day Yoho, I was fishing that area south of Port Hacking today. Not the best day only about 5m vis & 16C but a few small kingfish are starting to make an appearance & in a month or so, hopefully their bigger kin will move south also.
I really wouldnt change much about the gear I've used in regards to my own journey. I had been snorkelling & spearfishing for several years already when in my mid teens I was helped by a previous Australian Champ to make a speargun & have been making most of my own since. Even mistakes I have made along the way have all taught me something & I am happy to learn from these.
I'd suggest you get a gun about a 100cm first up, its on the small end of the scale for kingfish & the like but will be more than fine for reef species like morwong, bream, ludrick, drummer, etc. As for brands, there really isnt many bad guns on the market from the big name brands, dont go overboard at first, find something that you think you will like & go out & use it. If you find faults, great, you must really be using it, & then you can customise it a bit or choose another gun with a better understanding this time of what you need. I can promise you that if you really get into spearfishing you will have a quiver of specialist spearguns eventually.
May I suggest you check out the USFA web site. This is our peak spearfishing body & we have four clubs just in the Sydney region & a monthly competition between them, even if you dont want to fish comps, you would be welcome to attend club meetings, social events etc. Joining a club is also the fastest way to get good at shooting fish.