Not currently in any parties or orgs since the socialist left in Australia is close to non-existent and I've never felt that my joining any of them would do anyone any good. These are the ones I've personally interacted with, but there are more I'm not familiar with at all.
Tendency: Marxist-Leninist
basically the inheritors of the old cold war left here after the old CPA dissolved when the USSR fell and these guys (formerly Socialist Party of Australia) took on the name. Pretty typical M-Ls, focused on trade unionism/old fashioned labour activism, Aboriginal rights, anti-imperialism and all the usual stuff. Good people, very friendly and welcoming, skew pretty old. Still have connections with diplomats etc. from the few existing socdem or ML states (Venezuela, Cuba) and it's worth going to one of their Politics in the Pub events in the major cities, especially when they host an interesting guest speaker
Tendency: Orthodox Trots more or less
If you've ever read wooswoos you know what these guys are like. Incredibly sectarian, really intense, a little creepy sometimes. Habit of having their analysis scoffed at as pessimistic or self-serving only to later turn out to be correct.
Tendency: Trots, theoretically
Not affiliated with the CWI like SAlt USA. I think they might be or were associated with IST? Not sure. Hated by and hostile to pretty much everyone else on the left in Australia. I've only really encountered them at demos where they shout over everyone else and are almost as bad for pointlessly antagonising everyone as anarchists. Official line rejects "identity politics", on the ground seem to be interesting in nothing but. Used to be (still is?) a poster here who was a member for a short while and described cult-like behaviour, but I've never been close enough to see anything like that for myself. Accuse everyone else of being "middle-class parties" when they themselves consist of almost nothing but middle-class uni students.
Can you tell I can't stand these cunts?
Tendency: Trots maybe, demsocs maybe, greenies maybe, who the fuck knows
Hippies, de-facto Greens voters who are a little more radical and micronationalists usually end up at least on the edges of this group. They seem nice enough, have a couple of local city councilors elected here and there. Flit from campaign to campaign depending on what particularist issue hot at any given time and mostly treat socialism as an afterthought. Encountered them in the wild a couple of times and they seem like nice people but there's only so many white guys with dreads and bohemian uni chicks in hemp shawls I can stomach. If you're at a demo and looking to score some grass, find the Socialist Alliance guys and they'll sort you out.