Fair work has a whole section about PPL, have a look there are there is a lot of info. We have a policy on it at work and provide staff with a lot of information about it too so that they know what their entitlements are and what is required.
I don't really see it as Centelink's fault as such (other than not giving you the right info to start with) there are rules in place for a reason otherwise it would be possible fore people to take serious advantage of this system. However I agree that it doesn't seem fair that someone could get some casual work and still be eligible but someone who finish up work earlier but has been full time isn't.

Did you finish early for health reasons? This is one of the exemptions.