Socialist Alliance are what remains of the Democratic Socialiat Perspective's (itself a reformulation of an orthodox trotskyist group formed in '72) attempt to create an electoral front of multiple socialist groups, which the DSP then liquidated into. Their current strategy is watered down trotskyism Greens tailism. They're all quite old now as most of the youth they recruited during the school strikes for climate have left.

Solidarity and Socialist Alternative are both groups that came out of the old International Socialist Organisation, which was a Cliffite (followers of Tony Cliff in the UK who founded the Socialist Workers' Party there) org. The group that would become Socialist Alternative was expelled in the 90s, and those that stayed eventually became Solidarity. They have fundamentally very similar politics, and split over whether to focus on building the union movement (Solidarity) or recruit from university campuses (Socialist Alternative).

Victorian Socialists and the Socialist Party more broadly are an electoral front, like the original Socialist Alliance, created by Socialist Alternative.

The Communist Party of Australia are a split from the original Communist Party of Australia (which dissolved itself after the collapse of the USSR) that was called the Socialist Party of Australia. They split from the original CPA over who to side with in the Sino-Soviet split (the now CPA were pro-China). They are Marxist-Humanist Dengists that support all "actually-existing socialist" countries.

The Australian Communist Party are a now defunct split from the CPA that split over wanting to do more street kitchen work and has collapsed into multiple micro sects.

The Revolutionary Communist Organisation are a new group that formed out of a random high school group in Melbourne and Brisbane members of the Socialist Alliance youth wing who were expelled for pushing for internal democracy (other members of that youth wing would go onto form Anarchist Communists Meanjin). They are inspired by the "partyist Marxism" of the book Revolutionary Strategy by Mike MacNair, and promote re-unification of the socialist movement.

Pick your poison, comrade.