One of the arguments I have heard is that WorkChoices led to a relative decline in wages in some industries (specifically hospitality) while mining, for example, used WorkChoices to increase productivity.
Source: ABS Labour Price Index.
The labour price index shows wage changes for people performing the same work. It shows that wage increases in hospitality (or accommodation and food services, the nearest LPI category) were indeed low in the year after WorkChoices (which started in March 2006) compared to all industries and particularly mining. Accommodation and food services has a history of low increases and the post-WorkChoices year was not the worst for workers. However the fact that it was well below average for 2006-07 is consistent with WorkChoices having an effect.