The centralised approach involves government and their tribunals determining wage rates and employment conditions for industries applicable nationwide, and was used from 1904-1990s. An Award is created, which sets out the employment conditions and also contains the 10 National Employment Standards.
Basically, because of this, you have a legal document which sets out the conditions for an entire industry (e.g air pilots) rather than being focused on say each organisation.
Decentralised approach is the main method used today, and involves employers and employees (or unions representing them) going through enterprise bargaining (i.e negotiating conditions), to come to a decision and establish a collective agreement. This document will set out the wage rates and employment conditions for a specific organisation (e.g National Woolworths Supermarkets Agreement). Awards are still present in the decentralised approach, however they basically act as a 'safety-net' below collective agreements.
Hope that makes sense.