Shortages of labour due to the Victorian gold rush from 1851, the creation of responsible government which increased representation in 1857, the British Trade Union Act of 1871 and the 8 hour day campaign all contributed to a climate that fostered the Trade Union movement and the Trade Union Act of 1876. South Australia became the first territory of the empire outside Britain to legalise trade unions. The purposes of any Trade Union shall not, by reason merely that they are in restraint of trade, be deemed to be unlawful so as to render any member of such Trade Union liable to criminal prosecution for conspiracy, or otherwise [The Trade Union Act, 1876 (SA) s 2].
For more information see Radical dream, Union movement and The Foundation of South Australia: 1852-1883, Key events and issues.