Complex sentence: Almost all unions in Australia are affiliated with the Australian Council of Trade Unions (ACTU), many of which have undergone a significant process of amalgamations, especially in the late 1980s and early 1990s.
Split into simpler sentences: Almost all unions in Australia are affiliated with the Australian Council of Trade Unions (ACTU). These unions are commonly the product of a significant process of amalgamations, especially in the late 1980s and early 1990s.