The answer to this is simple and it is mainly to do with policies on industrial relations.

The Liberal party is pro business and sets laws that prevent workers from uniting to get better wages.  By forcing workers to bargain individually and giving the boss the power to lock you out or not employ you if you don't agree to his terms, workers are simply forced to accept whatever the boss offers.

This is ok for professional people who can call their own shots, but is dismal for non professional people who do not have a market advantage over other hopefuls.

The Labor party on the other hand promotes a level bargaining field by encouraging workers to bargain collectively, because it is only on a collective basis that you can be effective in negotiations, and relies on the Industrial Commission to make sure that things are above board on both sides.

Lets face it, what is it that a Union does?  It unites workers to bargain collectively so that the boss is forced to negotiate with the whole workforce instead of picking you off one by one.  Workers who are on AWA's know this better than anyone else.