The ISO formed in 1971 as the MWG, then as the Socialist Workers' Action Group (SWAG), and finally the International Socialists (IS), becoming the official representative of the IST in Australia.
Another period of internal crisis beginning in 2001 led to a loss of members and a further split in 2003 when another grouping of members around former leader Ian Rintoul left to form a group known as Solidarity.